Many, many thanks to the setter for a very enjoyable puzzle.
The clue of the day for me was 15dn. I didn’t realise “kipper” could be a verb. 4dn deserves a mention for its unusual structure!
Clues are in blue, with definitions underlined. Answers are in BOLD CAPS, then wordplay. (ABC*) means ‘anagram of ABC’. Explanations and deletions are in [square brackets].
1 Person trying to nab another’s second drink (6)
CHASER: a double definition. I’ve only underlined the second, more straightforward definition, since that’s the one used in 4dn. I slowed myself down on this clue because after I saw the “S”, CASSIS stuck in my mind from a recent puzzle.
5 On-field evidence of underground move to purge players? (8)
WORMCAST: WORM [to purge – e.g. to worm a dog, I suppose], CAST [players].
9 Means of transport connected works with college (8)
UNICYCLE: UNI is the college, the CYCLE is a set of connected works.
10 It's said writer will express tentative interest (6)
NIBBLE: sounds like [“it’s said”] the “nib will”, or the “nib’ll”!
11 Monarch's heartless companion put out (6)
QUENCH: QU[e]EN, CH.
12 Bishop's said no different church must be included (8)
DIOCESAN: [anagram: “different”] of (SAID NO*), “including” CE. Setters use an apostrophe-s in many different ways. Here it’s part of the definition.
14 Unorthodox setters, each vastly superior (7,5)
STREETS AHEAD: [anagram: “unorthodox”] of (SETTERS*), then A HEAD (“each”, as in “tuppence a head”).
17 Worshippers go on creating havoc (12)
CONGREGATION: [anagram: “havoc”] of (GO ON CREATING*).
20 Extend series of organs, etc, having expert as leader (8)
PROTRACT: PRO [expert], TRACT [series of organs etc – for example, the digestive tract].
22 Quietly judge team returning for initial qualifier (6)
PREFIX: P [quietly], REF [judge], IX=XI [team] “returning”.
23 Enters the lists with nothing out of place? Exactly (4,2)
JUST SO: JOUSTS, with the O moved “out of place”.
25 Warship conquistador's docked taking surgeon aboard (8)
CORVETTE: CORTE[z], “taking aboard” VET.
26 Eponymous hero, graduate in a woman's clutches (4,4)
ADAM BEDE: the graduate is a B.Ed., dutifully put into the “clutches” of A DAME.
Amazing to see this book two Saturdays in a row, when I’d never heard of it before. It was George Eliot’s first novel, apparently.
27 Devoted pet welcoming first of travellers home (6)
DOTING: first of T[ravellers] and IN [home], all “welcomed” in DOG.
2 Special period in which there's no integrity (6)
HONOUR: NO in HOUR. Easy once you see it, but made harder by a cunningly placed definition. Is “special” just a distraction? Not really – a particular hour might be special for some reason, as in “cometh the hour, cometh the man”.
3 Gift claimed by Sibyl back on location, we hear (6,5)
SECOND SIGHT: SECOND [back], SIGHT sounds like SITE [location], “we hear”.
4 1 across, say, or 1 across drunk by English (9)
RACEHORSE: since 1ac is CHASER, this is an anagram [“drunk”] of (OR CHASER*), followed by E [English]. I assume the definition refers to “steeplechaser”. It seems very unusual to have to go to another answer to get both the definition and the anagram letters!
5 Win over the French added to women's respect (7)
WHEEDLE: W [women’s], HEED [regard], LE [“the” in French]. It wasn’t easy to see whether the definition was “win over” or “regard”.
6 Repetitive movement from knocker on door (5)
RONDO: I often overlook hidden answers. D’oh!
7 Transport info for citizens provided here (3)
CAB: the CAB is transport, the C.A.B. is the Citizens’ Advice Bureau.
8 Sailor abroad set aside (4,4)
SALT AWAY: needs no explanation, I assume.
13 Boost canteen menu briefly with hot stew (11)
ENHANCEMENT: [anagram: “stew”] of (CANTEEN MEN- H*). “Briefly” warns us to drop the last letter of MENU.
15 Bowlers finally cured, as whole team needs to be? (9)
SKIPPERED: [bowler]S, KIPPERED. I didn’t know that KIPPERING is a process.
16 Avoided what host did in pub? (3,5)
GOT ROUND: double definition.
18 Specialist diplomat cut off offensive ambassador (7)
ATTACHÉ: ATTAC[k], H.E. [ambassador’s title]. An attaché can be a junior or a specialist diplomat – like a trade or cultural attaché.
19 Part of pump beginning to swivel stops mine working (6)
PISTON: S[wivel] in PIT ON.
21 Enlivened old king admitted to enormous dread (5)
AWOKE: O[ld] K[ing] in AWE.
24 Headgear provided by mum, temperature rising (3)
TAM: MA, T all “rising”.