Across:
1 Greatest footballer (Best = B-EST) grabs a (-A-) shirt at the top (‘shirt’ at the top = -S-) (BASEST) – it’s most improper (BASEST),
4 Get into the head, unnerving (PSYCH OUT) spy (spy = PSY-) corrupting (= anagram indicator) church (-CH) with common knowledge (OUT) (PSYCH OUT),
9 Paper (SUN) contacts creator (GOD) (SUN GOD) of The ’RA? (Ra = SUN GOD),
10 Huge celebrity (MEGASTAR) mars Gate (mars Gate = MEGASTAR) production (= anagram indicator),
12 Local official (TOWN CLERK) from Trim (TOWN) found in court (CLERK) (TOWN CLERK),
13 Wisecracker missing career (‘wisecracker’ missing ‘career’ = wisck = WICKS) in old hamlets (old hamlet/town = WICKS),
14 Imagine (PICTURE) film unit (FRAME) (PICTURE FRAME) in The House giving it one’s best shot perhaps (PICTURE FRAME),
18 Up (= reversal indicator) north (NI = IN-), leads intimate evensong naturally (leads intimate evensong naturally = -IEN) in religious community (convent = -CONVEN-T) (INCONVENIENT) – is that ill-timed? (INCONVENIENT),
21 One of The Doors (EXI-T) admits lead singer (lead ‘singer’ = -S-) (EXIST) could be alive (EXIST),
22 Confusion (MAELSTROM) produced by Moslem art (Moslem art = MAELSTROM) creation (= anagram indicator),
24 Take a break (REST-) in Waterford (-RING) (RESTRING) – it will add something to your bow (RESTRING),
25 Had leading role (HERO-) meeting Third Reich (third ‘Reich’ = -I-) and got caught (-C) (HEROIC) – how is that deserving of a medal? (HEROIC),
26 Too nervous (RESTLESS) to relax (REST-) on the foreign (the = -LE-) ship (-SS) (RESTLESS),
27 Spud’s getting fried (spuds/potatoes getting fried = CRISPS) in college (C-) by teacher (Sir = -RIS-) ramping up (= reversal indicator) pressure (-P-) and is starting to stress (starting to ‘stress’ = -S) (CRISPS).
Down:
1 Transport (BUS) strikes (STOPS) (BUS STOPS) in shelters at various stages (BUS STOPS),
2 Makes a meal (SANDWICH) of gritty stuff (SAND-) coming from women’s organisation (Women’s Institute = -WI-) connected to church (-CH) (SANDWICH),
3 Is patient (STOIC) with most of (k) the animals (‘stock’ without ‘k’ = STO-C) around island (-I-) (STOIC),
5 Informer (SNEAK) gets to post (P-) critical appraisal (-REVIEW) (SNEAK PREVIEW) and the pundit will get to see it (SNEAK PREVIEW),
6 Board member (chairwoman) loses one (i) (‘chairwoman’ without ‘i’ = CHARWOMAN) member of the cleaning staff (CHARWOMAN),
7 Policeman (cop) loses head (c) (‘cop’ loses ‘c’ = OP-) with jerks (-TICS) (OPTICS) hanging out of the bar (OPTICS measure drinks/spirits in bar),
8 End product (end ‘product’ = T-) is harsh (harsh = -HRASH), shapeless (= anagram indicator) (THRASH) leather (leather/beat = THRASH),
11 New issues (DEVELOPMENTS) to improve (DEVELOP-) worker’s (workers = -MEN-) shirt (-T- shirt) with top-stitching (top ‘stitching’ = -S) (DEVELOPMENTS),
15 Strange (UNNATURAL) organisation (UN-) is unaffected (-NATURAL) (UNNATURAL),
16 Plot (BED-) and digs (-ROOMS) (BEDROOMS) aimed at retiring types (BEDROOMS),
17 Butcher’s (= anagram indicator) costs – ham (costs ham = STOMACHS) or pot bellies? (STOMACHS),
19 Fear and trembling (TERROR) on first tee (first ‘tee’ = T-) with inaccuracy (-ERROR) (TERROR),
20 Pine for (MISS-) America? (-US) (MISSUS) Address it! (MISSUS is a form of address),
23 Lance (lance/spike = SPEAR) spins (= reversal indicator) artist’s (RA’s = S-AR) record (extended-play record = EP = -PE-) inside (SPEAR).