I enjoyed this, and I don't think it's too taxing. A couple of engineered anagrams, a tad of slightly obscure vocab, and some pleasingly corny homophones. I got repeatedly interrupted so I don't know how long it took - no more than 10min, I think. I look forward to the kicking I shall no doubt receive in the comments regarding my knowledge of Indian history.

Across
1	Noisy bird —type of spaniel also, some say (8)
	COCKATOO - sounds like COCKER, TOO. Arf
5	Assist with a B movie? (4)
	ABET - A + B + ET, saccharine 80's Spielberg film that I've never seen
8	Lie about Republican disharmony (8)
	FRICTION - FICTION outside R
9	Smile broadly, having right to dip into drink (4)
	GRIN - R inside GIN
11	Heartless mob in EU’s to board vehicle (5)
	EMBUS - Heartless mob is MB, with EUS outside. Rather soutenu word for 'get on a bus'
12	Shrub recollected in Gray’s Inn mostly (7)
	SYRINGA - Anagram ('recollected') of GRAYS IN, (i.e. most of GRAY'S INN)
13	Second go required to orbit a planet (6)
	SATURN - S + TURN around A
15	Steal from school, carrying can back (6)
	SNITCH - SCH with TIN backwards inside
18	Act like a shopaholic, so to speak — eventually (2,3,2)
	BY AND BY - Sounds like BUY AND BUY
19	Country featuring in main diagram (5)
	INDIA - hidden word: maIN DIAgram
21	Exchange prisoners from the east (4)
	SWOP - POWs backwards. Less familiar spelling of SWAP.
22	Workers securing sick leave at last in WI islands (8)
	ANTILLES - Workers are always ANTS (unless they are BEES, or TU for trade union, or MEN, or HANDS). Insert ILL  and E (end of LEAVE)
23	Unfeeling Greek character, one with medical qualification (4)
	NUMB - NU is the Greek character, MB is the medical qualification.
24	Meanly swindle leaders of Ireland’s loyal youths (8)
	STINGILY - swindle is STING, + first letters of Ireland's Loyal Youths

Down
1	Reportedly people unpopular in theatre boxes? (7)
	COFFERS - Sounds like COUGHERS. Ha ha. I ran off on a massive wild goose chase to start with because of the word BOXES, which I though might be a verb. Overthinking it as ever.
2	Caught arm, perhaps, in mountaineering activity (5)
	CLIMB - C + LIMB
3	Travelling most of Saturday to a Continental motorway (10)
	AUTOSTRADA -  Anagram ('travelling') of  SATURDA (most of SATURDAY) + TO + A. Remember Curarist's Law: any word that one is instructed to shorten, is only ever reduced by one letter, unless it's specified (half, two thirds etc)
4	Player primarily on bench, or in second team? (6)
	OBOIST - Initial letters of On Bench, Or In Second Team
6	Titled man’s circle in London borough (7)
	BARONET - London borough is BARNET, with O inside. BARNET also means hair via Cockney rhyming slang (Barnet Fair = hair)
7	Knight wearing Roman garment in the Friendly Islands (5)
	TONGA - N for Knight (in chess) inside TOGA
10	Change criminal into saint? About right (10)
	TRANSITION - Anagram ('criminal') of  INTO SAINT + R
14	Players entertaining Aussie native in café (7)
	TEAROOM - Players are TEAM, Aussie native is ROO
16	Tries always to identify gossip (7)
	HEARSAY - Tries (in court) is HEARS, AY is always.
17	Extremely clumsy snare trapping tail of young bird (6)
	CYGNET - Extremely clumsy is CY, snare is NET, with G (tail of younG) inside. Although a cygnet is a young bird, the definition here is just 'bird', because 'young' is part of the cryptic clue.
18	Pudding bowl, as found in rubbish receptacle (5)
	BASIN - AS inside BIN
20	Once a capital food shop, we hear (5)
	DELHI - Sounds like DELI. 'Once a capital'? hmm. Not sure that Delhi (as opposed to New Delhi) was ever capital of India. Before New Delhi was established, the capital was Calcutta. Am I right?

A well above average Jumbo, I felt, with the usual preponderance of straight-down-the-line clues pleasingly peppered on this occasion with a nutritious selection of &lits, reverse cryptics and just plain quirky meta-offerings. Many things in contention for COD, but I'll give my award to 35dn as I always enjoy the breaking of the fourth wall, if indeed crosswords can be said to have walls. I'll leave it to the readership (that's you) to enumerate some of the other splendid clues in this grid - many thanks setter, for some real cryptic belters!

ACROSS
1 Side holding power in a splinter group (6)
ASPECT - P [power] in A SECT [a | splinter group]

4 Underworld boss having mobile that goes on by itself (10)
PERSEPHONE - PHONE [mobile] that goes on PER SE [by itself]

10 Go to form new colony as part of vicious war, maybe (5)
SWARM - hidden in {viciou}S WAR M{aybe}

14 Getting hump somehow in ordeal is characteristic of success (9)
TRIUMPHAL - (HUMP*) ["somehow"] in TRIAL [ordeal]

15 Strange edict about one article after another penned by relative is shown to be true (13)
AUTHENTICATED - (EDICT*) about A [one article] after THE [another (arcticle] "penned" by AUNT [relative]

16 Refuse to admit learner that’s less developed (7)
LITTLER - LITTER [refuse] to "admit" L [learner]

17 On foot, it provides quick cover (7)
TOENAIL - cryptic def: quick as in the part of the foot under the nail.

18 Cruciverbalist’s last kind of wordplay, having put out third puzzle (7)
TANGRAM - {cruciverbalis}T + AN{a}GRAM [kind of wordplay, "having put out third" (letter)]

19 Avoiding offence, in a way — like copper, initially (11,7)
POLITICALLY CORRECT - referred to be the initials PC, just like a Police Constable is.

21 Behave like obsequious dog or other little animal (4)
FAWN - double def

24 Decide not to run with small old-fashioned weapon (5)
SPIKE - S PIKE [small | old-fashioned weapon]. As in "to spike an article".

26 Investor finally compensated, holding current issued shares (8)
RATIONED - {investo}R ATONED [compensated], "holding" I [current]

27 Make from sale, having also included promotional activity (8)
BRANDING - BRING [make from sale], having AND [also] included

29 Result of making name for oneself as fighter (3,2,6)
NOM DE GUERRE - cryptic def

30 What can be got from what’s in menu, in short (11)
NOURISHMENT - (MENU IN SHORT*) ["what can be got from what's in..."], &lit

32 Rock group worth breaking up? Not so far (6,5)
STONE'S THROW - STONES [rock group] + (WORTH*) ["breaking up"]

35 Cut back — result of freeze, say — in authoritarian regime (6,5)
POLICE STATE - reversed LOP [cut] + ICE STATE [result of freeze | say]

37 Respite, as European is accepted by British fairly (8)
BREATHER - E [European] is accepted by B RATHER [British | fairly]

39 Bad fortune besetting European — score reduced by six (8)
FOURTEEN - (FORTUNE*) ["bad"] "besetting" E [European]. Score as in "twenty".

40 Something outstanding about university’s appearance initially (5)
DEBUT - DEBT [something outstanding] about U [university]

43 What’s the matter with intelligence? Poet’s pronouncement (4)
GREY - homophone of GRAY (Thomas, 1716-1771, of Elegy Written In A Country Churchyard fame). Grey matter is "the matter with intelligence".

44 It’s half a world away from where we are here (8,10)
SOUTHERN HEMISPHERE - cryptic def. Rather assuming that the solver is not in Australia!

47 Improperly dispensed upscale form of medication (7)
CAPSULE - (UPSCALE*)

48 Fruit was revolting with it (7)
ROSEHIP - ROSE [was revolting] + HIP [with it]

50 King protected by subjects in heated situations (7)
TROPICS - R [king] "protected" by TOPICS [subjects]

51 Enjoying, for example, cricket? Over count is repeatedly revised (13)
INSECTIVOROUS - (OVER COUNT IS IS*) ["revised"]

52 Fifteenth century house in city (9)
LANCASTER - double def

53 In peculiar way, how swindler is selected for side? (5)
ODDLY - reverse cryptic of S{w}I{n}D{l}E{r}

54 Full of amazement since a lot of weight I lost (10)
ASTONISHED - AS TON I SHED [since | a lot of weight | I | lost]

55 Oblivion achieved ultimately in drink in this? (6)
BENDER - {oblivio}N {achieve}D, in BEER [drink], semi-&lit

DOWN
1 Wild animals run into stakes (9)
ANTELOPES - LOPE [run] into ANTES [stakes]

2 Painters initially list oil in mixed medium art technique (11)
POINTILLISM - P{ainters} + (LIST OIL IN*) ["mixed"] + M [medium]

3 Left in piece of jewellery on time for royal court (7)
CAMELOT - L [left] in CAMEO [piece of jewellery] on T [time]

5 Report written up about century, brilliant effort (5)
ECLAT - TALE [report] reversed about C [century]

6 House of Lords, perhaps, having ethos tamely changed (7,4)
STATELY HOME - (ETHOS TAMELY*)

7 Confuse pagan with theory of ancient philosophy (11)
PYTHAGOREAN - (PAGAN + THEORY*)

8 See way to support past master, above all (8)
OVERLORD - LO RD [see | way] to support OVER [past]

9 Motor vehicle, say, in competition overturned (6,3)
ESTATE CAR - STATE [say] in reversed RACE [competition]

10 Behind one's back (6)
SECOND - double def

11 Later adjusted temperature in simple option (11)
ALTERNATIVE - (LATER*) ["adjusted"] + T [temperature] in NAIVE [simple]

12 Subject of most original palindrome? (5)
MADAM - I think this is a cryptic reference to "Madam, I'm Adam", allegedly the first palindrome spoken, in the Garden of Eden after the creation of Eve.

13 Current malfunction in LA? (5,7)
SHORT CIRCUIT - reverse cryptic, as a LA{p} is a "short" circuit.

20 Scholarly American's volume found on a railway (8)
LITERARY - LITER [American's volume] found on A RY [a | railway]

22 Female article of clothing close, along with male one (7)
NIGHTIE - NIGH [close], along with TIE [male (article of clothing)]

23 Motivated in this way with drug, in conclusion (8)
ENTHUSED - THUS [in this way] + E [drug], in END [conclusion]

25 Take part in novel event held after ten (8)
ELEVENTH - hidden in {nov}EL EVENT H{eld}

28 One seeing what’s going to be charged for sweet (5,3)
BULL'S EYE - if you're about to be charged by a bull, the thing seeing what's going to be charged (i.e. you) is the bull's eye!

29 What contains primarily oats, say, easily being consumed by horse? (7)
NOSEBAG - O{ats} S{ay} E{asily} B{king}, "consumed" by NAG [horse], semi-&lit

31 Popular with constituents, including female relative that’s erratic (12)
INCONSISTENT - IN [popular] with CONTENT [constituents], "including" SIS [female relative}

33 Given too much publicity about European vote presented (11)
OVEREXPOSED - OVER E X POSED [about | European | vote | presented]

34 Golf clubs holding English Open, finally — competitors don’t want it (6,5)
WOODEN SPOON - WOOD & SPOON [golf clubs] "holding" E [English] + {ope}N

35 Reduce new academic work that immediately follows this (11)
PARENTHESIS - PARE N THESIS [reduce | new | academic work]. The parenthesis follows the clue, containing its enumeration!

36 Moderated about one part of speech being held up, in short (11)
ABBREVIATED - ABATED [moderated] about reversed I VERB [one | part of speech]

38 Specialized publications I create so cryptically (9)
ESOTERICA - (I CREATE SO*)

41 Manager of funds changed rate with more confidence (9)
TREASURER - (RATE*) ["changed] + SURER [with more confidence]

42 Sailor aboard sturdy launch (5,3)
START OUT - TAR [sailor] "aboard" STOUT [sturdy]

45 Liable to embrace sweet girl, initially, being vulgar (7)
PROFANE - PRONE [liable] to embrace F{anny} A{dams} (as in "sweet Fanny Adams")!

46 Forceful or pitifully weak about check (6)
PUNCHY - PUNY [pitifully weak] "about" CH [check]

47 Capital in company affected by inflation? (5)
CAIRO - CO [company] "affected by inflation", which is to say, filled with AIR

49 Cry holding son giving sign of life (5)
PULSE - PULE [cry] "holding" S [son]