Across:
1 Examines in a nosy way perhaps (SNIFFS), setting up (= reverse indicator) party (Sinn Féin/SF = -FS) for those on the bass (fins on bass fish = fins = SNIF-) (SNIFFS),
4 Ban (BAR-) ringers (-BELLS) (BARBELLS) for those putting on weight (BARBELLS),
9 Booze (RUM-) cruise (tour) lacks leadership (t) (‘tour’ without ‘t’ = -OUR) (RUMOUR) or that’s the word on the street (RUMOUR),
10 A real gin (a real gin) cocktail (= anagram indicator) (a real gin = ALGERIAN) produced by a resident of Africa (ALGERIAN),
12 Samples (PORTFOLIO) drink (PORT-) with duck (-O-) in filo (filo) bake (= anagram indicator) (filo = F-LIO) (PORTFOLIO),
13 Digging up (= reverse indicator) turf (sod = -DOS) for the neighbours’ (UK = KU-) (KUDOS) pats on the back (KUDOS),
14 They put on masks (BEAUTY SALONS) for Grace (grace/appeal = BEAUTY) and most of (e) those isolated (alone) (‘alone’ without ‘e’ = ALON-) in the ship (SS = S-S) (BEAUTY SALONS),
18 Has a job examining relative’s (FAMILY DOCTOR) bloodline (FAMILY) for Spike (spike/tamper = DOCTOR) (FAMILY DOCTOR),
21 Collect (AMASS) those in Mount Ossa mad (those in ‘mount oSSA MAd’ = ssama) to return (= reverse indicator) (ssama = AMASS),
22 Leading back (leading ‘Back’ = B-) row (-ROW-) triumphs over (-BEATS) (BROWBEATS) bullies (BROWBEATS),
24 Endless (f) foam (‘surf’ without ‘f’ = SUR-) on pints of stout (-ROUND) (SURROUND) in Ring (SURROUND),
25 Turns up (= reverse indicator), for example (e.g. = -GE), to study (den = -N ED-) at Oxford (O-) (ON EDGE), a bundle of nerves (ON EDGE),
26 Wild (= anagram indicator) rascals (rascals = AR- CLASS) admit starting trouble (starting ‘Trouble’ = -T) (ART CLASS) and in that you’ll get to recognise a familiar picture (ART CLASS),
27 Throwaway lines (ASIDES) from The Undertones on stage (ASIDES).
Down:
1 Have just enough in one's locker (SCRAPE BY) to beat (= anagram indicator) pacy Serb (pacy Serb = SCRAPE BY),
2 One (I-) Frenchman (Monsieur = -M-) is a type of enemy (-MORTAL enemy) (IMMORTAL) here to stay perhaps (IMMORTAL),
3 Make a mess (FLUFF) of closing (‘oF’ closing = F-) large (-L-) youth centre (‘yoUth’ centre = -U-) for party (Fianna Fáil = -FF) (FLUFF),
5 Nothing decided yet (ALL TO PLAY FOR), as gangster (Al Capone = AL-) gets lot (lot) wrong (= anagram indicator) (lot = -L TO)? Oppose (PLAY) with backing (FOR)! (ALL TO PLAY FOR),
6 The summer holidays presumably (BREAK) for the teachers (Irish National Teachers’ Organisation = INTO) (BREAK INTO) and Rob (BREAK INTO),
7 Those from the stable (lads = LA-DS) going over the heads of idealistic, rich (heads of ‘Idealistic Rich’ = -IR-) (LAIRDS) overseas landowners (landlords in Scotland = LAIRDS),
8 School (S-) with a (-A-) treehouse (-NEST) (SANEST) has the most marbles perhaps? (SANEST),
11 Flexible (ELASTIC) characters in 23 down (‘bands’ = BANDS) (ELASTIC BANDS) holding everything together (ELASTIC BANDS)
15 Oscar (-O-), Bill (-AD-) and Old Bob (old ‘bob’ = shilling = -S-) as well (-TOO-), taken in by retiring (= reverse indicator) lieutenant (lt = T-L) (TOADSTOOL), recognised by the cap in the forest by mycologist? (TOADSTOOL),
16 Description of Robinson Crusoe (STRANDED) found on the shore (STRAND-) by writer (-ED) (STRANDED),
17 Rugby player (prop) cutting (p) (‘prop’ without ‘p’ = PRO-) back on (= reverse indicator) gym units (reps = SPER-) somehow initially (‘Somehow’ initially = -S) (PROSPERS) does well (PROSPERS),
19 Plant (CASSIA) producing dope (-ASS-) occupying agency of spies (C-IA) (CASSIA),
20 Turns up (= reversal indicator) on The Hill (tor = -ROT) for the music (rap = PAR-) (PARROT) and is likely to repeat everything you say (PARROT),
23 Bars (bars/lines = BANDS) for the musical types (BANDS).