Thank you to Filbert. Definitions are underlined in the clues.
Across
1. Bachelor travels on motorway to help girl taking train (10)
BRIDESMAID : B(abbrev. for “Bachelor”) + RIDES(travels, say, on a train) plus(on) M(abbrev. for “motorway”) + AID(to help/assist).
Defn: …, lifting the train of the bride’s wedding dress, that is.
7. Thump boyfriend admitting condition (4)
BIFF : BF(abbrev. for “boyfriend”) containing(admitting) IF(a condition/stipulation).
Defn: …/hit with the fist.
9. Collaborator who French support when injured (8)
QUISLING : QUI(French for “who”) + SLING(a piece of cloth that supports one’s injured arm).
Defn: One who collaborates with an enemy force occupying one’s country.
10. Mother curtailed baby’s insubordination (6)
MUTINY : “mum”(a familiar term for one’s mother) minus its last letter(curtailed) + TINY(informal term for a very young child/a baby).
11. Sceptic bishop feeds tailored version of God to humanist (8,6)
DOUBTING THOMAS : B(abbrev. for “bishop” in chess notation) contained in(feeds) anagram of(tailored version of) GOD TO HUMANIST.
13. Would it be OK if I said “cocktail“? (3,3)
MAI TAI : Homophone of(… said) “Might I?”(“Would it be OK if I …”, as in “Might I do this?”).
15. Nurse leant on a roll (8)
ENLISTED : EN(abbrev. for “enrolled nurse”, in the UK) + LISTED(of a ship, inclined/leant to one side).
Defn: …/put on a register.
16. Silence, a lack of records? (3,1,4)
NOT A PEEP : NO(a lack/absence of) TAPE,EP(two recording and playback audio formats, the former a magnetic strip, and the latter an “extended play” vinyl disc).
19. Certainly, carrying knife one goes in for sharpness (6)
ACUITY : AY(a positive response/”Certainly!”) containing(carrying) [ CUT(to stab/to knife) containing(… goes in) I(Roman numeral for “one”) ].
Defn: … of mind.
21. Ambassador among others here at four dealing with English sandwiches (14)
REPRESENTATIVE : [ PRESENT(is among/with others here, at this particular place) + AT + IV(Roman numeral for “four”) ] contained in(… sandwiches) [ RE(with regard to/dealing with) + E(abbrev. for “English”) ].
23. One standing at the side of the room by work buffet (6)
WALLOP : WALL(the vertical structure/one standing at the side of the room) plus(by) OP(abbrev. for “opus”, a long musical or literary work).
Defn: …/a heavy blow.
25. Happy turning barge right, one managed easily (8)
PUSHOVER : Reversal of(… turning) UP(in high spirits/happy) + SHOVE(to barge/push roughly) + R(abbrev. for “right”).
26. Neighbours’ greeting, perhaps not straight across front of drive? (4)
G’DAY : GAY(not straight/not heterosexual) containing(across) 1st letter of(front of) “drive”.
Answer: In Australia and New Zealand, a friendly/neighbourly greeting, a variant of “good day”.
27. Flask in bag right at the bottom (10)
NETHERMOST : THERMOS(a flask/container that keeps fluid hot or cold) contained in(in) NET(to capture/to obtain).
Down
2. One piece of toast ample? (5)
ROUND : Double defn: 2nd: …/plump, descriptive of a person’s body.
3. Revolt suits leaders of guerrillas desperate for reform (7)
DISGUST : Anagram of(… for reform) [SUITS + 1st letters, respectively, of(leaders of) “guerrillas desperate” ].
Defn: …/to repulse.
4. Aren’t ropes aboard ship very good quality? (11)
SAINTLINESS : [ AIN’T(non-standard form of “aren’t”) + LINES(ropes/cords) ] contained in(aboard) SS(abbrev. for “SteamShip”, used in the names of such).
5. WWI battleground Nivelle evacuated with gas coming over (7)
ARGONNE : “Nivelle” minus all its inner letters(evacuated) contained in(with … coming over) ARGON(an inert gas).
Answer: …, in north-eastern France.
6/24. Restrain mother hen (6)
DAMPEN : DAM(mother/female parent of an animal) + PEN(a female bird, in this case, a swan).
7. Remote controls, absolutely loads (7)
BUTTONS : BUT(merely/absolutely, as in “It’s but a fancy of his”) + TONS(a large quantity/loads).
8. Kitty met an awful arse (9)
FUNDAMENT : FUND(a pool of money/kitty) + anagram of(… awful) MET AN.
Defn: …/the buttocks.
12. Florida city high-rise enjoys view across area (11)
TALLAHASSEE : [TALL(high-rise/of great height) + HAS(experiences/enjoys) + SEE(to view/watch ] containing(across) A(abbrev. for “area”).
14. American shot by Fritz, perhaps earlier (9)
AFOREHAND : A(abbrev. for “America”) + FOREHAND(a shot by a tennis player, an example of whom/perhaps is American Taylor Fritz).
Defn: …/prior.
17. Recover gold crowns by listening (7)
AURALLY : RALLY(to recover/get better) placed below(… crowns, in a down clue) AU(symbol for the chemical element, gold).
18. Container of two halves, name seen in mine with lid flipped (4,3)
PINT POT : N(abbrev. for “name”) contained in(seen in) PIT(a deep hole from which stones and minerals are dug/a mine) plus(with) reversal of(… flipped, in a down clue) TOP(a lid/a cover).
Defn: …/two half-pints of beer or similar drink.
20. Showing equality, further education to benefit millions (7)
UNIFORM : UNI(short for “university”, further/higher education) + FOR(given to/to benefit the recipient) + M(abbrev. for “millions”).
22. Half chance rugby doesn’t start (5)
EVENS : “sevens”(or seven-a-side, a form of rugby union involving teams of seven players each) minus its 1st letter(doesn’t start).
Defn: … of either winning or losing.
24. See 6
Succinctly à propos surfaces, as per for Filbert; that for Doubting Thomas was a ripper. For buttons I was thinking something like “But of course!” Hmm, either way, not easy to substitute … But great puzzle, thanks F and scchua.