Across:
1 Reviews (= reversal indicator) bed and breakfast (B&B = -BB-) in great hall in Greece (Stoa = A-OTS) (ABBOTS) directly over a number of religious buildings (ABBOTS),
4 Not (= anagram indicator) sure (sure = USER-) luminary (-NAME) (USERNAME) provided a surfer’s identity (USERNAME),
9 A lot missing from alternator (‘a lot’ missing from ‘alternator’ = ternar = ERRANT) for going places (ERRANT),
10 Outfits (UNIFORMS) inform (inform = -NIFORM-) criminal (= anagram indicator) in America (U-S) (UNIFORMS),
12 Alternatively (OR-), revolutionary’s (Che’s = -CHES-) art (art = -TRA) review (= reversal indicator) (ORCHESTRA) is producing the numbers (ORCHESTRA),
13 One of those at the bar (Senior Counsel = SC-) on the beer (-ALE) (SCALE) is appreciated by musician (SCALE),
14 Take a breath (SIGH-) on ridiculous (=anagram indicator) gradient (gradient = -T-READING) (SIGHT-READING) taking to the stage without any preparation (SIGHT-READING),
18 Describes obsessive type (ONE-TRACK MIND) on (ON-) the last horse (last ‘horse’ = -E-) at The Curragh? (-TRACK) Watch out! (MIND) (ONE-TRACK MIND),
21 Frenchman (Messieurs = -M-) in part (part = TRA-P) upset (= reversal indicator) (TRAMP) by hike (TRAMP),
22 Ongoing (INCESSANT) instances (instances = INCESSANT) all over the place (= anagram indicator),
24 Sailor (able-bodied seaman = AB-) with wicked weakness (-SIN-) for starter of trout (starter of ‘trout’ = T-) with hot (-H-) English (-E) (ABSINTHE) liqueur (ABSINTHE),
25 One of those in 1 down (‘ale’ from ‘alehouse’ = ALE-) has father (parish priest -PP-) from old (-O) (ALEPPO) city in Syria (ALEPPO),
26 Allergic reaction (RASH) to passion? (HEAT) For starters (= position indicator) (HEAT RASH), you’ d get that from The Sun! (HEAT RASH)
27 Olive and Laurel (types of GREENS) take data from Great Danes (take ‘data’ from ‘Great Danes’ = grenes = GREENS).
Down:
1 Pub (ALEHOUSE) auction (sale) doesn’t include top (s) (‘sale’ without ‘s’ = ALE-) accommodation (-HOUSE) (ALEHOUSE),
2 Robin (BIRD-) and Pen (pen/coop = -CAGE) (BIRDCAGE) has a hold on the Twitterati perhaps? (BIRDCAGE),
3 Sent (sent = TENS-) off (= anagram indicator) for Spain (-E) (TENSE) in a state of agitation (TENSE),
5 Drama (SONG AND DANCE) witnessed in Oklahoma (Oklahoma musical is a SONG AND DANCE),
6 Dissident (REFUSENIK) scrap (rubbish/refuse = REFUSE-) with Ulster (Northern Ireland = -NI-) on the weekend (‘week’ end = -K) (REFUSENIK),
7 Tar (able-bodied seaman = AB-) sticks on avenue (-ROAD) (ABROAD) in France, for instance (ABROAD),
8 English (E-) flower (-ASTER) (EASTER) is a spring annual that follows some hard and fast rules (EASTER),
11 Courses (STREET-S) about Pale (LIGHT-) (STREET LIGHTS) provides illumination (STREET LIGHTS),
15 Lifts the lid off (TIN-OPENER) troubled (= anagram indicator) intern (intern = TIN -NER) grabbing Frank, briefly (‘open’ = -OPE-) (TIN-OPENER),
16 Bitter humour (BI--LE) embodies great (-G-) programme (‘programme’ = ‘program’ = -APP-) on the radio (= homophone indicator) (BIG APPLE) in American city (BIG APPLE),
17 Issues (EDITIONS) upsetting (= reversal indicator) leaders of North of Ireland tourism (leaders of ‘North of Ireland tourism’ = noit = -TION) divides Ulster, for one (side = EDI -S) (EDITIONS),
19 Tie up (ATTACH) that (that = ATT-H) mess (= anagram indicator) overshadowing a (-A-) college (-C-) (ATTACH),
20 Use an (use an = NAUSE-) anomaly (= anagram indicator) to go to a (-A) (NAUSEA) medic to diagnose this (NAUSEA),
23 Somebody outrageously starts (‘somebody outrageously’ starts = SO-) knocking back (= reversal indicator) most of (e) the drink (‘ale’ without ‘e’ = al = -LA-) and reading (-R) (SOLAR) from The Sun (SOLAR).