Across:
1 Foolish figure (ASS-) first (1ST = -IST) (ASSIST) to help (ASSIST),
4 One of those working in a public house (TAP) gets drink (WATER) (TAP WATER) that’s tasteless and ends up down the drain (TAP WATER),
9 A crude container (BARREL of crude oil) full of wine (BARREL),
10 Rough (= anagram indicator) partisan (partisan = ASPIRANT) has ambitions (ASPIRANT),
12 Outfit for one of those retiring types (NIGHTGOWN) of dudes leaving shotgun wedding (‘dudes’ leaving ‘shotgun wedding’ = hotgnwing = NIGHTGOWN),
13 Subject (THEME) to helpmate getting rid of pal (‘helpmate’ getting rid of ‘pal’ = hemte = THEME),
14 Imagined (ALL IN THE MIND) doing a (A) couple of lines (-LL) at home (IN) out of one’s head perhaps? (THE MIND) (ALL IN THE MIND),
18 Shooting up (GROWING) leads to lots of suffering (PAINS) (GROWING PAINS) for those developing while doing a stretch (GROWING PAINS),
21 Has left Art House (‘has’ left ‘art house’ = rtoue = ROUTE), by the way (ROUTE),
22 One of the big guns made this impression (SHELL HOLE) coming out of the sand (SHELL) on the golf course (HOLE) (SHELL HOLE),
24 Stop trying (JACK IT IN) to quit? (JACK IT IN),
25 Criticises (KNOCKS) sounds from The Doors (KNOCKS),
26 Opencast mining loses impact (‘opencast mining’ loses ‘impact’ = oensning = NEON SIGN) in the dark – commercially, it sheds light on what needs to be sold (NEON SIGN),
27 Got involved in the plot (WEEDED) and ousted the Scarlet Pimpernel, for example (ousted the ‘scarlet pimpernel’ weed = WEEDED).
Down:
1 Living in Europe (ALBANIAN) with a (A-) lab (lab = -LBA-) engineer (= anagram indicator) and Ulster (Northern Ireland = -NI-) supporter (fan) needing no introduction (f) (‘fan’ without ‘f’ = -AN) (ALBANIAN),
2 Give it one’s best shot (STRUGGLE) with horse (-GG-) in Ulster (Ulster = STRU-LE) run (= anagram indicator) (STRUGGLE),
3 Was the Princess and Pea expected to lie about this (SHEET on a bed) in the paper? (SHEET in the newspaper),
5 Not in school (ABSENT-) to look after (-MIND-) and feed half (‘feed’ half = -ED) (ABSENT-MINDED) of those described as daydreaming (the ABSENT-MINDED),
6 Composed (WRITTEN) and awake (UP) (WRITTEN UP) or so the story goes (WRITTEN UP),
7 Go on the road with The Tourists (TRAVEL) and see the world (TRAVEL),
8 One of the ranks (rank smell = ROTTEN) is corrupt (ROTTEN),
11 Get rid of the dirt on the suits, for example (DO THE WASHING), in party (DO) and most of them (most of ‘them’ = THE) joining that shower (WASH-) in (-IN-) Government (-G) (DO THE WASHING),
15 Reinsures (reinsures = NURSERIES) building (= anagram indicator) for Junior & Co (NURSERIES),
16 Victor (-V-) or (-OR-) Charlie (-C-) lost power in the car (DI-ED) outside (DIVORCED) of Split (split = DIVORCED),
17 Has the measure of (ASSESSED) fool (ASS-) with contaminated (= anagram indicator) seeds (seeds = -ESSED) (ASSESSED),
19 Content of Metro – January’s (content of ‘Metro – January’s’ = tro jan = TROJAN) Man with reputation for hard work (TROJAN),
20 Centurions get no rest ('centurions’ get no ‘rest’ = cnuion = NUNCIO) on papal mission (NUNCIO),
23 Spike (LANCE) Lancaster dismisses the arts (‘Lancaster’ dismisses the ‘arts’ = lncae = LANCE).