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.