As always, I will endeavour to provide an complete and error free account of the workings of this puzzle, and leave the rest of you to spot where I have missed the mark.
1 SACCHARIN sweetener
Ghastly stuff, C7H5NO3S to its very few friends, on my palate vying with aspartame for truly nasty aftertaste. Bag: SAC, tea: CHAR, in: -um- IN.
6 APIAN social worker described thus
It's the other social worker in a crossword this time, the humble bee. Quiet gives you the musicain's PIANO which is curtailed by one letter after you have spotted that the A at the beginning of the clue is not just there for decoration.
9 RAMADAN Religious festival
Is festival the right word? It is, after all, mostly a month of fasting and general abstention. Here, it's managed: RAN surrounding A + cross: MAD. Extra points for religious probity if you can read the word out loud without automatically following it with "ding dong".
10 TOPPING custard
Chambers gives "to finish off; to dispose of finally" for polishing off, so a reasonable if not immediately obvious synonym for "topping". If you normally put the custard in first, of course, you might want to argue that topping it ain't. Be my guest.
11 PODGE One with pounds to spare
The word "say" gives you EG, which you reverse and tack on the end of POD, a school of whales, and not YSGOL which is a school in Wales, possibly Betws y CoEd
12 FIRECREST Bird
The musician's loud is F, one is I, others is/are REST, and the park is a REC(reation ground). Gettable from wordplay, and a guess as to what it might look like is probably right.
13 SAWBONES Doctor
It helps if you know that BS is a Bachelor of Surgery. Here it surrounds ONE for "a certain person" and follows SAW for "consulted". Sawbones shortened to Bones for Leonard McCoy.
14 HEAP Old car
Not worth much is CHEAP. Fall the front off.
17 NOTE Message
The even letters of iN bOtTlE
18 EUROCRAT Brussels employee.
You can't make sprout picker fit, so make do with firm: CO(mpany), regret RUE, tax: RATE. Reverse the first two in tandem, add a cut version of the third.
21 GREAT AUNT Relative
Here's where my world fell apart as I entered a Bristolian GREY TAUNT. Amazing how hard it is to spot the error. the ultimate in complaininG, about: RE, a: A, gibe: TAUNT.
22 TACIT understood.
The food found in short supply is TACo, it's IT
24 GORDIUS very old ruler
You know him from the knotty problem solved at a stroke by Alexander the Great. Attempt: GO, straight line: RADIUS, remove the A.
25 RAILMEN On-line workers (tee hee)
Hidden (though not very well) in winteR AILMENTs
26 LEHAR Musician
A "reinterpreted" Mahler missing the first letter.
27 DUDE RANCH holiday resort
California, form which you only need the initial C, helps to point you towards this US style vacation spot. Surround the C wth an anagram (gathering) of A HUNDRED
1 SCRIP There might have been money in it
An old word for a traveling bag, cf Luke 10.4 in the KJV "Carry neither purse, nor scrip, nor shoes", and here "to write for TV" or SCRIPT contracted by one letter.
2 COME DOWN TO EARTH face reality
Wake up provides COME TO, sad gives the "nursed" DOWN and a broken heart is EARTH if you break it in the right way.
3 HEDGEROW Shrubs
Have no fear, thiis is not some green excrescence you've never heard of. "Fellow would cultivate" gives HE'D GROW, into which you insert you insert the E found at the end of cottage. See what I mean about smooth cluing?
4 RUN AFTER pursue
Rugby, or the 15 a side version anyway, is RU, A supporter is FAN, which is reversed, and a short period is TER(m). Assemble.
5 NOTARY Legal official
A(rea) surrounded by NO TRY for no go.
6 ASPECT viewpoint
A faction give A SECT (once again, don't ignore the A). Insert P from P(olitical) leader.
7 IF I WERE A RICH MAN song
OK, here we go. On Internet connection gives RE WIFI, and that's "turned". Another version of H(ot) AMERICAN is EARICHMAN, which when correctly spaced gives Tevye's big number. Mrs Z went to see FotR with her dad shortly after revealing she wanted to marry a Goy. Oy oy oy!
8 NIGHTSPOT Disco
Which it might well be. Near: NIGH, Main: TOP, Street: ST. Revererse the last two in tandem and add to the first.
13 SYNAGOGAL Like religious edifice
The select-one-out-of-seven note is G, some is ANY, and the Nigerian port is the one you've heard of, LAGOS. Tuck two into one and reverse the lot.
15 BUTTERED covered with spread.
Complete gives you UTTER, which you insert into the odd letters of BrEaD. Is it me, or are there rather o lot of insertion clues today?
Lettuce see, do we know of any other Greek islands of sufficient brevity? COS, its bank a TIER, the pound an L, appropriately inserted. That word again.
19 ÉTRIER rope ladder
For use on mountains. Energy E, and person testing is a TRIER. Add accents to choice. If you can.
20 BUSSED transported by coach
Picked up is not a soundzlike indicator, but another reversal instruction, in this case of DES, French "from the" if you ignore the grammar, and SUB, warship.
23 TENCH Fish
The unknown number is N, and the school a TECH. Insert (aaargh!) one into the other.