Not much to say about this.  It was perfectly acceptable, occasionally witty, fare (albeit with a couple of rather odd CDs), taking around 35 minutes.

First in was MISINFORM at 1a, last was STALL.

Clues are in blue with the definition undelined.  Anagram indicators are in bold italics.

Notation:

DD: Double definition

CD: Cryptic definition

DDCDH: DD/CD hybrid where a straight definition is combined with a cryptic hint.

&Lit:  "all in one" where the entire clue is both definition and wordplay.

(fodder)* denotes an anagram of the letters in the brackets.

Rounded brackets are also used to add further clarity

Squiggly brackets {} indicate parts of a word not used

Deletions are struck out

Square brackets [] expand an abbreviation or shortening like N[orth]

Across
1	Head of Sixth leaving teacher in class to teach wrongly? (9)
	MISINFORM - MISS IN FORM with S{ixth} removed
6	Fat cat running courses (7)
	CROESUS - (courses)*.  Some rich king from somewhere ancient.
10	Passage in tabloid is rejected (5)
	SINUS - IN SUN reversed
13	Information not available about what constitutes place of punishment (7)
	GEHENNA - GEN, N/A around EH?  Precicely what form of punishment you'd get here depends on which religion you follow.  Wikipedia helpfully advises us not to confuse it with Gahanna, Ohio, where they probably object to Kings of Judah sacrificing their children by fire at the golf academy.
14	Block digital protection (5)
	STALL - DD.  A finger stall wasn't at the forefront of my mind so this was last to fall.
15	A criminal catching disease being loaded (9)
	AFFLUENCE - A FENCE around FLU
16	TV ad that'll cause some to switch over to the other side (5,9,9)
	PARTY POLITICAL BROADCAST - CD
17	Bird having ring ouzel's head and eagle's tail (6)
	HOOPOE - HOOP, O{uzel}, {eagl}E
18	Device giving go-ahead for soldiers to penetrate area that's been shelled (3,5)
	EGG TIMER - CD
19	Paint a pack of hounds with vermin cut off (7)
	ACRYLIC - A, CRY, LICe.  I didn't know that a CRY was a pack of hounds but knowledge of the paint was enough to assume that it must be.
22	Scruffy pair? Mend it and press (5,5)
	PRINT MEDIA - (pair mend it)*
23	No right to get Chemistry, say, without one's submission (12)
	ACQUIESCENCE - ACQUIrE SCiENCE
27	Waste from metal symbolically rendered (5)
	OFFAL - OFF, AL[uminium]
29	Mobile or mikes for bugging (7)
	IRKSOME - (or mikes)*
30	Not initially fleeced, smaller lamb will be (8)
	HORNLESS - sHORN, LESS. Some might he unhappy that there's no "what a" "..." or "this" to complete the definition but I was fine with it
32	Idle fellow entertaining religious scholar with his belief system (8)
	BUDDHISM - BUM around D[octor of] D[ivinity], HIS
34	A racehorse with energy in first place, one hears (7)
	EARDRUM - A RED RUM with the E[nergy]moved to the front
36	Cleo cut short halfway through snake's embrace (5)
	CLASP - CLeo, ASP.  I liked the combo of Cleo and the ASP
39	Not a landowner — one with under a dozen workers? (6,6)
	TENANT FARMER - He's only got ten ants.  Geddit?
41	Artist’s endless row with journalist ultimately in newspapers (10)
	SCULPTRESS - SCULl, {journalis}T in PRESS
44	Potter receiving stick for Black going down? (3,4)
	CUE BALL - Whilst the surface works in terms of Harry Potter and Sirius Black the question mark is doing an anful lot of work to get the "literal" to work.  At a stretch a cue ball "receives" the stick (cue) then hits and pots the black but the "potter" is surely the player rather than the ball?  Very odd.  I spent a while trying to make B = black in some clever wordplay but don't think I'm missing anything.
46	Sap parasites returning to infest wood, edges chewed off (8)
	IMBECILE - More LICE (have we got an infestation?), this time reversed in tIMBEr
48	No longer healthy to produce emissions (6)
	EXHALE - EX, HALE
50	Web firm resolved perverse incentive to rid content of art (8,7,8)
	INTERNET SERVICE PROVIDER - (perverse incentive to rid)*, {a}R{t}
53	Pony tour down to exclude beginners? That'll be without me! (2,4,3)
	ON YOUR OWN - pONY tOUR dOWN.  Neat.
54	Useless writer held back by sex (5)
	INEPT - PEN reversed in IT
55	Append final note — one of these? (7)
	ADDENDA - ADD, END, A, semi &Lit
56	Quaint end of old cloth (5)
	TWEED - TWEE {ol}D
57	Unconscious after drink, having to leave society? (4,3)
	DROP OUT - OUT after DROP
58	Soldier to change sides twice (6,3)
	DESERT RAT - DESERT, RAT

Down
1	Study of foolish character on horse (3,2)
	MUG UP - MUG, UP
2	Swingball in one’s backyard? (6,2,9)
	SPHERE OF INFLUENCE - Hmm.  Another CD where the question mark is doing a lot.  Swing is influence, a ball is a sphere, ones sphere of influence might extend to ones backyard...
3	Over-protective, vigorously criticise butter (5,4)
	NANNY GOAT - NANNY (as in state), GO AT
4	Speaker setting out opposing sides of argument between two alternatives (6)
	ORATOR - A{rgumen}T in OR, OR
5	Very old relative's problem with web page? (7,4)
	MISSING LINK - DDCDH, and a good one at that.
6	Things once drawn from events worse than Boston tea party? (8)
	CHARIOTS - CHA RIOTS.  Boom-boom
7	Give bribe, perhaps, for source of cosmetic ingredient (3,4)
	OIL PALM - DDCDH
8	Nasty smell between cliff and unpleasant seaside resort (11)
	SCARBOROUGH - B[ody] O[dour] in SCAR, ROUGH
9	Stifle vocal county rowing team (9)
	SUFFOCATE - homophone of Suffolk Eight
10	Vibration from milk production centre, quiet at first (7)
	SHUDDER - UDDER after SH
11	Killer policeman in Jamaica is hiding (5)
	NINJA - hidden
12	Departs after show with rings round the eyes? (10)
	SPECTACLED - D[eparts] after SPECTACLE
17	Fruit on Italian flower, one that's large and fleshy (5)
	HIPPO - HIP on PO
20	Outside design contractor secures head, up-and-coming Frenchman with pull (9,8)
	LANDSCAPE GARDENER - LANDS, CAPE then RENE DRAG reversed.  I asked a landscape gardener to come and give me a quote for some work but he said he couldn't help me as my garden is portrait.
21	House of stone in India (6)
	GEMINI - GEM, IN, I[ndia]
24	Fruit for woman to shout about (6)
	CHERRY - HER in CRY
25	Mostly hit it off, embracing love in gut reaction (5)
	COLIC - CLIC{k} around O
26	Leave me out of summons, to be consistent (6)
	COHERE - COme HERE
28	Young man being featured regularly in Loaded (5)
	LADEN - LAD, {b}E{i}N{g}
31	Run, walk, walk (6)
	RAMBLE - R[un], AMBLE
33	Mounted strike, getting unexpected rise — enthusiastic about gains via union (7,4)
	MARRIES INTO - RAM reversed, (rise)*, INTO
35	Remembered to gather in grass (11)
	RECOLLECTED - COLLECT in REED
37	Group of friends sit around smoking in van (5)
	POSSE - POSE around S{moking}
38	Don't give up project to introduce computers (5,2,3)
	STICK IT OUT - STICK OUT around I.T.
40	Deception of one pretending to be criminal? (9)
	FALSEHOOD - DDCDH.  A pretend criminal might be a false hood.
42	Socialist, the first character to get cross at elevating one cheating exchequer (3,6)
	TAX EVADER - reversal of RED, A, VEX, AT
43	Wrong blood group supplied to crash in West End? (8)
	OCCIDENT - ACCIDENT with O replacing A
45	A good circuit — but you won't be able to move after running it! (7)
	AGROUND - A G[ood] ROUND
47	Our Brit cooking that's eaten in Mexico (7)
	BURRITO - (our Brit)*
49	Wetland thoroughfares (6)
	BROADS - B ROADS.  Never been to the Norfolk Brods, really need to put that right.
51	Your old setter, one going to pot? (5)
	THYME - THY, ME
52	Tripe keeping when cooked a certain way (5)
	ROAST - ROT around AS

I enjoyed this a lot. It seemed very fresh, and potent as well. There were several CDs, all very amusing; no (usually either dull or unfathomable) DDs; a not-untimely reference to a particular conjuncture in US-Russian relations; an invitation—to me, at least—to some historical/etymological edification (clue 18): and even a term (also US-centric) that reminded me, and in an agreeable way, of my own backwoods background, as well as of my all-time favorite singer (the song was at first, at least, more associated with Joan Baez, though she didn’t write it… either).

I do (armganas)* like this, and italicize anagrinds in the clues.

ACROSS
1	Old actress predicted modern movie formats (5,5)
	BETTE DAVIS — BETTED (“predicted”) + AVI (Audio Video Interleaved), plural. Anyone got Bette Davis AVIs?
7	One opening in long black robe (3)
	ALB — ”One” is A, then L[-ong] and B(lack). The L is clued this way because it’s not a conventional abbreviation for “long,” whereas B is conventional for “black.”
9	From its reading, novel completely changed (14)
	TRANSMOGRIFIED — (from its reading)*
11	Sporty husband into tactile dancing (8)
	ATHLETIC — (tactile)* with H(usband) inserted
12	Chief’s friend grabbing skirt (6)
	PRIMAL — P(RIM)AL
14	Lack of infrared type of film (4)
	NOIR — No I.R.
15	Reagan’s proposal to make love not war? (4,6)
	ZERO OPTION — CD. Yes, indeed, in 1981 US President Reagan proposed the withdrawal of all Soviet and United States intermediate-range nuclear missiles from Europe, and—although at this juncture anti-nuke activists accused him of making the offer only in full expectation of its being rejected, thereby giving him an excuse to up the ante in the arms race—there’s actually a good chance he was sincere. It was as if he had never realized what nuclear weapons could do until after he became president (which is not hard to believe). A few years later, after Gorbachev came to power in the USSR, serious nuke-reduction treaties were signed. (Oh, yes, those were the days…)
18	Fellowship found in unexciting marches (10)
	BORDERLAND — B(ORDER)LAND. This sense of “marches” does not derive from the fact that dividing lines between, say, England and Scotland or Wales have often been embroiled in conflict and thus have been patrolled by armed forces (my first thought!); “march” in this sense is derived from “mark,” originally meaning a boundary, or the sign of a boundary (and eventually only a sign). (Although the sense of walking purposefully and/or in step with others derives directly from an Old French term for “trample,” even that word may originate in a Frankish term meaning “mark.”) Did you know that the words “marquess,” “marchioness,” et “marquis” all derive from this sense of “march”? I didn’t!
19	Light brown part of pie crust (4)
	ECRU — Hidden
21	Small village managed 50 per cent of chores (6)
	RANCHO — RAN + CHO[-res]. I haven’t found support for this definition. Wiktionary says that in Spanish it means, “properly, a mess, mess room,” and gives two American definitions: a small hut or a “large grazing farm.” In Merriam-Webster, the first def. is simply “ranch.” I know there can’t be some odd UK alternative! There are towns with this word in their name, however—por ejemplo, Rancho Velos, in Cuba. Jackkt found the definition in the SOED, which I have not at hand.
23	Puzzling item opened by bank (8)
	THROWING — TH(ROW)ING. Cambridge: “A bank is also a row of similar objects: A bank of computers displays the latest weather data.”
25	Underwear for mum? (14)
	UNMENTIONABLES — CD
26	Complex parts with radius taken in (3)
	HAD — ”Complex” being “hard,” which lets go of its R(adius)
27	Rebel soldier in group is heading for trouble (10)
	SEPARATIST — SE(PARA)T + IS + T[-rouble]

DOWN
2	Stay inside weaving rather more material (9)
	EARTHLIER — LIE inside (rather)*
3	Fiddle one considers not hard (6)
	TINKER — T[-h]INKER
4	Miss one’s opening of French wine (10)
	DEMOISELLE — DE (of, Fr.) + MO(I)SELLE
5	Very mature rising star (4)
	VEGA — V + AGE<=
6	Bones touch and move the wrong way (8)
	STIRRUPS — STIR (“touch”—as in being, uh, moved emotionally) and “move,” SPUR<=
7	Saw a cross on an island (5)
	AXIOM — A + X + IOM (Isle of Man)
8	That imparted by a figure of speech? (4,8)
	BODY LANGUAGE — CD. A “figure” can be a “body,” which can express itself more or less eloquently
10	Artist realises old river branch is crossing (12)
	GAINSBOROUGH — GAINS, “realises” (in the financial sense) + B(O)(R)OUGH
13	Old criminal, still running? (10)
	MOONSHINER — CD. I learned when on vacation with my teetotaling parents many years ago that I had two great-uncles in Pennsylvania who used to run stills. Bob Dylan (who purveys some fine whiskey himself these days) is singing “Copper Kettle” in my head as I type. “We ain’t paid no whiskey tax / Since 1792.” The only time I’ve ever drunk moonshine was in the early 1970s at a bluegrass festival on a hill near my hometown—I was just so bored by the twanging banjos after a half-hour… I don’t remember much else about that night.
16	Wound in chest is something irritating (9)
	ITCHINESS — (in chest)*
17	My opinion is old-fashioned (8)
	METHINKS — CD. Now-quaint way of saying IMO
20	Ring, perhaps one created by Tolkien (6)
	HOBBIT — On top of an oven you will find a HOB, with a "ring" being a BIT of it.  [Previous entry: HOB is “ring” (a heating coil on top of an oven; I didn’t find a dictionary that defined it as a ring, but a picture is worth a thousand words), and BIT is “perhaps one” (the other OPTION being ZERO) Well,, that was fun while it lasted.]
22	Called me in and exploded (5)
	NAMED — (me in and)*
24	I’m initially placed under wide doormat (4)
	WIMP — W(ide) + IM + P[-laced]