Across:
1 No wall in stairwells (no ‘wall’ in ‘stairwells’ = stires = SISTER) or in the abbey (SISTER),
4 Civil rights leaders (‘civil rights’ leaders = CR-) from central Magdeburg (central ‘Magdeburg’ = -E-) and Cologne (-SCENT) (CRESCENT) found on estate (road/street of housing estate = CRESCENT),
9 Forcibly remove (UPROOT) our top (our top = UPROOT) model (= anagram indicator),
10 Local hooligan (yob) loses the head (‘yob’ without ‘y’ = OB-) with one of those in court (-SERVER on tennis court) (OBSERVER) – presumably, it’s the witness? (OBSERVER),
12 Season (PEPPER-) on (-ON-) island (-I) (PEPPERONI) goes down well with the Italians (PEPPERONI),
13 Subject (THEME) to helpmate getting rid of pal (‘helpmate’ getting rid of ‘pal’ = hemte = THEME),
14 Snack (OPEN SANDWICH) for Frank (OPEN) Bacon, for instance (Bacon SANDWICH) (OPEN SANDWICH),
18 Times angered (times angered = DISAGREEMENT) organisation (= anagram indicator) with difference of opinion (DISAGREEMENT),
21 Stay away from City Hall’s (‘stay’ away from ‘City Halls’ = cihll = CHILL) unfriendliness (CHILL),
22 Can’t complain about (NO EXCUSES) that negative response (NO) to why it went wrong (EXCUSES)? (NO EXCUSES),
24 Line (COR-D-) about most of (p) the last letters (‘RIP’ without ‘p’= -RI-) from old (-O-) writing (from the 3Rs = -R) (CORRIDOR) passage (CORRIDOR),
25 Back (= reversal indicator) one (-I) graduate (BA = -AB-) with motto (motto/proverb/saw = WAS-) (WASABI) – a really hot one! (WASABI),
26 Humpty Dumpty (EGGHEAD-) goes to school (-S) (EGGHEADS) with local clever people (EGGHEADS),
27 The problem with flying (JET LAG) plane (JET) to American city (LA-) for golf (-G) (JET LAG).
Down:
1 Someone who knows the shape of things to come (SCULPTOR) makes an impression on one of The Stones (SCULPTOR),
2 Swinging back around (= reversal indicator), photographic device (lens = S- NEL) captures bang (bang/thump = -RAP-) and hot (-H-) (SHRAPNEL) fragments from bombshell (SHRAPNEL),
3 Call up (EVOKE) first lady (EV-E) over fine (fine = okay = -OK-) (EVOKE),
5 Bunny (RABBIT) Warner (Warner = WARREN) working (= anagram indicator) (RABBIT WARREN) in Buck House (RABBIT WARREN),
6 Pat Eccles (Pat Eccles = SPECTACLE) recovers (= anagram indicator) sight (SPECTACLE),
7 Endive (endive = ENVIED) salad (= anagram indicator) to die for (ENVIVED),
8 Heartless (n) old jailer (‘turnkey’ without ‘n’ = TURKEY) is expected for Christmas (TURKEY),
11 Arabian (Arabian horse = MOUNT-) and American (-A-) from up (= reversal indicator) North (NI = -IN), by the way (ROAD) (MOUNTAIN ROAD), going around the Sugar Loaf (MOUNTAIN ROAD),
15 Even now (STILL) old magazine (LIFE) (STILL LIFE) is put out by the artists (STILL LIFE),
16 Dinner (ME- -AL) includes starters of salads, seafood, hams (starters of salads, seafood, hams = -SS H-) and libations opened (and ‘libations opened’ = -L-) (MESS HALL) in private dining room (MESS HALL),
17 Concealing (STASHING) fraudulent enterprise (sting = ST-ING) outside? Academy (-A-) is calling for silence! (-SH-) (STASHING),
19 Hanging around freezing (ICICLE) with couple of those in charge (in charge = IC x 2 = ICIC-) of The Spanish (the = el) Review (= reversal indicator) (el = -LE) (ICICLE),
20 It’s well (OIL RIG) for those doing the boring stuff (OIL RIG),
23 Set one’s heart on (CRAVE) enthusiastic praise (-RAVE) for college (C-) first (= position indicator) (CRAVE).