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.
Thanks to both. I assumed Neighbours was a reference to the Australian TV series.
I also felt the blog should have mentioned the soap ‘Neighbours’. In 21a, ‘among others’ should be part of the definition.
Personally, I felt 20d should have said ‘higher education’. This is distinct from ‘further education’ which is, essentially, at the same level as school but at greater depth and mainly performed at colleges.
When I first read 15a, I noticed that NEONATAL was an anagram of ‘leant on a’ but I already had the E from ARGONNE.
My first run through revealed very little and I thought I was in for a long slog. However all gradually emerged with no recourse to the reveal button.
Possibly there is more of a general adjectival synonym involved with TINY at 10a. Tiny meaning baby in other senses than an informal word for a child, e.g. baby grand, baby carrots etc.
I enjoyed this as I do most Filberts and thanks to scchua too.
I have never seen a female swan defined as a hen before. I know Chambers says a hen is a female bird and so is a pen but defining one as the other is pushing it somewhat.
Otherwise, usual solid fare from the prolific Filbert. Thanks both.