Thank you to Filbert. Definitions are underlined in the clues.
Across
1. Baby group starts with some trigonometry (6)
COSSET : SET(a collection of similar items/group) placed after(starts with) COS(abbrev. for “cosine”, a function in/some trigonometry).
Defn: To ….
4. Hit with tear gas from behind, quietly scarper (6)
DECAMP : Reversal of(… from behind) MACED(sprayed/hit with Mace, a brand name for a self-defence spray containing tear gas) + P(abbrev. for “piano”, a direction to play part of a music piece softly/quietly).
8. Clashing plate host eats on (10)
DISHARMONY : DISH(a plate holding food) + ARMY(a large number of people, animals, and things/a host) containing(eats) ON.
9. Help island cutting bananas (4)
MAID : I(abbrev. for “island”) contained in(cutting) MAD(insane/bananas).
Defn: Household …

10. Three types of buffet show indecision (4,3,3,4)
BLOW HOT AND COLD : [ BLOW(a hard hit/buffet) + HOT(a type of buffet/a self-service meal of hot food laid out on a table or counter) AND COLD(same as the hot buffet, but serving cold or room temperature food) ](three types of buffet).
11. Underground shelter arrived at with input from Republican (6)
GROTTO : [GOT TO](arrived at/reached one’s destination) containing(with input from) R(abbrev. for a Republican).
13. Not drinking after a dreadful experience with old liqueur (8)
AMARETTO : TT(abbrev. for “teetotal”, not drinking/abstaining from alcohol) placed after(after) [A + MARE(a dreadful/unpleasant experience) ] plus(with) O(abbrev. for “old”).
Answer: An Italian liqueur.
15. Prisoner muddled memo that’s clear for Spooner? (8)
CONSOMME : CON(short for “convict”/a prisoner)’S + anagram of(muddled) MEMO.
Defn: That (soup) which is clear served to/for a spooner/cryptically, one using a spoon.
17. Hebrew bible Ruth can attempt in part, reading backwards (6)
TANACH : Hidden in reversal of(… in part, reading backwards) “Ruth can attempt”.
19. Jockey in bar over Ascot where there are track-side views (11,3)
OBSERVATION CAR : Anagram of(Jockey) IN BAR OVER ASCOT.
Answer: A specialised train carriage designed for passenger sightseeing, in other words from where there are railway-track-side views.

22. Infection small yet spreading (4)
STYE : S(abbrev. for “small”) + anagram of(… spreading) YET.
Defn: … of the eye.
23. God’s will supporting study into sin (10)
PROVIDENCE : PRO(in favour of/supporting) + [ DEN(a study/room affording privacy) contained in(into) VICE(an immoral habit/a sin) ].
24. Exclusive clubs get rid of head of diversity (6)
CLOSED : C(abbrev. for “clubs”, a suit in a deck of playing cards) + LOSE(to get rid of/to discard) + 1st letter of(head of) “diversity”.
25. Sinister type didn’t eat filling in pies (6)
LEFTIE : LEFT(didn’t eat) + middle 2 letters of(filling in) “pies”.
Defn: One who is on the political left or “sinister”, the Latin word for “on the left-hand side”.
Down
1. Sportsperson physically flexible and cold at the top (7)
CLIMBER : LIMBER(having supple joints and muscles/physically flexible) placed below(and … at the top, in a down clue) C(abbrev. for “cold”).
2. Scot mostly takes His Majesty for a fool (5)
SCHMO : “Scot” minus its last letter(mostly) containing(takes) HM(abbrev. for His Majesty).
Answer: American slang for a fool.
3. Ground elder’s back – what, more soil improver? (9)
EARTHWORM : Anagram of(Ground) [ Last letter of(…’s back) “elder” + WHAT, MORE ].
Answer: An organism whose activities in its soil habitat enhances/improves the health and fertility of the soil.
4. Senior member aching after party (5)
DOYEN : YEN(a yearning/an aching for) placed below(after, in a down clue) DO(a social function/party).
5. One half of Macclesfield stupidly admit criminal activity (4,5)
COME CLEAN : Anagram of(… stupidly) [ONE + 1st 6 out of 12 letters of(half of) “Macclesfield” ].
6. Jersey put in the post with sale item (7)
MAILLOT : MAIL(delivered letters and packages /post) plus(with) LOT(item for sale in, say, an auction).
Significant jerseys:

7. Carrie having squeezed out four, mechanism to limit that? (13)
CONTRACEPTION : “Carrie” minus its 4 inner letters(having squeezed out four) contained in(… to limit that) CONTRAPTION(a mechanism/device).
Defn: A device to limit/prevent a woman, possibly one named Carrie, from having another baby after having given birth to/squeezed out four babies. A possible Carrie is Mrs. Boris Johnson, who has 4 children.
12. Naff stuff primarily in gallery? Not so (9)
TASTELESS : 1st letter of(… primarily) “stuff” contained in(in) TATE(one of four art galleries in the UK) + LESS(to a smaller degree or quantity/not so …, as in “not so hard”).
14. Agree to gold even if more money (9)
AUTHORISE : AU(symbol for the chemical element, gold) + THO’(contraction of “though”/even if/notwithstanding) + RISE(an increase in amount/more of, say, money, as in “price rise” or “pay rise”).
16. M25 maybe spoken about little (7)
ORBITAL : ORAL(spoken as opposed to written) containing(about) BIT(a little/small portion or amount of something).
Defn: An example of which is/maybe the M25, a motorway that loops in a large circle around Greater London.
18. Hide boat with start of cyclone forecast (7)
CORACLE : 1st letter of(start of) “cyclone” + ORACLE(a divine message telling the future/a forecast).
Defn: A small boat possibly made from animal hide.

20. 12‘s fast, making the top five (5)
VAPID : “rapid”(fast/quick) with its 1st letter replaced by(making the top, in a down clue) V(Roman numeral for “five”).
Defn: The answer to 12 down.
21. Male cat has caught rat, perhaps (5)
CHEAT : HE(third-person pronoun for a male) contained in(… has caught) CAT.
Defn: An example of/perhaps a despicable person/a rat.
I liked the not Spoonerism and the slur on the Cheshire town. The usual high standard from Filbert and scchua.
Oddly enough I thought 7d was CONTRACEPTION when I only had one crosser and had no idea why, and still didn’t when I got the other crossers and wrote it in. Nice puzzle, some great surfaces and misdirection.
Great stuff from Filbert as always.
Couldn’t work out what was going on with CONTRACEPTION which was contraceptive for a while just because it had the Roman four in it.
CONSOMMÉ was last in with me being befuddled by the spooner and wondering whether it was kosher to be able to spoonerism an anagram.
Liked the neat anagram of OBSERVATION CAR although I incorrectly assumed it was referring to the Citroens that used to circle the outside of the track at the same time as the horses.
I did one go on a CORACLE making course when I had money and time to waste before children. Thankfully, those ones were hide free.
Ticks for many more as well.
Thanks Filbert and schuaa
Filbert always delivers! Overall I thought it was at the easier end of his spectrum, but I did fail on the intersecting CORACLE (nho and IMO unguessable; clever surface with plenty of scope to mislead on the wordplay) and LEFTIE (sneaky definition; nothing to do with politics, I feel, but rather the literal definition of sinister as left-hand).
CONTRACEPTION came about in the same way as for Staticman1@3, and in fact I never did parse it properly, though the wordplay is impeccable; another clever, misleading clue there.
My podium consists of CONSOMMÉ for not being a Spoonerism (it was hard to see from the surface grammar how it could really be one, but the PDM was great even so), BLOW HOT AND COLD, and AMARETTO for the excellent surface.
Thanks both
A great puzzle to end the week on
CONTRACEPTION was my LOI as it was a word that fitted and when I parsed it and discovered it was correct and an &lit I was very impressed
My fave clue however was CONSOMME as it’s only the second time I’ve seen a clue mentioning Spooner that has Spooner as the definition (or in this case, part of it) – the first time I believe the answer was CUDDLER (and it might’ve also been a Filbert clue but I can’t remember)
Thanks Filbert and Scchua