Across:
1 In retrospect (= reversal indicator), research site (lab = -BAL-) divides Frenchmen (MM = -M-M) initially eager (initially 'Eager' = E-) (EMBALM) to use preservatives in a deadly way (EMBALM),
4 Extraordinary (STRANGE-) writing (one of the 3Rs = -R) (STRANGER) in Alien (STRANGER),
9 Segment of enclave neglected (segment of ‘enclAVE NEGlected’ = aveneg) around (= reverse indicator) (aveneg = GENEVA) lake in Switzerland (Lake GENEVA),
10 Aesthetically pleasing (TASTEFUL) soft spot (TASTE-) for fellow (-F-) at university (= University of Limerick = -UL) (TASTEFUL),
12 Being unlikely to boast (HUMBLE), detective (PI-) found last clue (last letter of ‘cluE’ = -E) (HUMBLE PIE), those feeling the shame consumed it (HUMBLE PIE),
13 Some characters in Ottawa keeping (some characters in ‘ottAWA KEeping’ = AWAKE) up (AWAKE),
14 Organises (= anagram indicator) the races (the races = TEACHERS) for dogs (PETS) (TEACHER’S PETS) – the classy favourites (TEACHER’S PETS),
18 Get to provide direction (CONTROL) for The Office’s (ROOMS) (CONTROL ROOMS) units keeping surveillance on the video feed (CONTROL ROOMS),
21 Small group (CLUMP) of Iberians dismissed by Republicanism (‘Iberians’ dismissed by ‘Republicanism’ = pulcm = CLUMP),
22 Describe the current state of affairs (ENLIGHTEN) in form? (inform = ENLIGHTEN),
24 One of those on the phone (RECEIVER) dealing with what’s currently hot (RECEIVER),
25 A high-pitched reaction (SCREAM) to last of the reds (last of the ‘redS’ = S-) coming in contact off white? (off-white = -CREAM) (SCREAM),
26 Place (-PUT-) in Waterford (The Déise = DE-ISE) (DEPUTISE) is under study? (understudy = DEPUTISE),
27 Crows (BOASTS) and snakes (BOAS-) found around The States (found around ‘The stateS’= -TS) (BOASTS),
Down:
1 It's perfect for meringue (EGG WHITE), for example (e.g. = EG-), and for shifting (= anagram indicator) weight (weight = -G WHITE) (EGG WHITE),
2 A dog’s dinner perhaps (bone meal = BONEMEAL) for those who know their onions and what helps them grow (BONEMEAL fertiliser),
3 Most of those closing 11 down (most of those closing ‘spirit levels’ = LEVEL) in flat (LEVEL),
5 Exchanges (TRADES-) gym (PE-) for pole (pole) dance (= anagram indicator) (pole = -OPLE) (TRADESPEOPLE) by skilled workers (TRADESPEOPLE),
6 A (A-) retweet (-RT) about (= reverse indicator) Sam (Sam = MAS-), Lawrence (-TE- Lawrence) and Regina (-R) (ART MASTER) getting students to draw a veil over something perhaps (ART MASTER)
7 Roar with laughter (GUFFAW) in Washington (WA = -AW) at party (Fianna Fáil = -FF-) for uncompromising banks (‘UncompromisinG’ banks = ug = GU-) all getting backed (= reverse all indicator) (GUFFAW),
8 It’s a comfort (RELIEF) to official (RE-F) catching story (-LIE-) (RELIEF),
11 They’re perfect for an evening out with bubbles (SPIRIT LEVELS perfect for evening out/checking something is level with the ‘bubble’) and the barman will be aware of them if he’s done a stocktake? (SPIRIT LEVELS),
15 Run off (-PRINT) after (= position indicator) local dance (HOOF-) (HOOFPRINT) or that’s the impression the Arabian made (HOOFPRINT is the impression the Arabian horse made),
16 Brendan Grace (BOTTLER-) and the ultimate comics (ultimate ‘comicS’ = -S)? (BOTTLERS) Presumably, if you stole from them, you’d be lifting their spirits! (BOTTLERS fill spirits/drinks into bottles),
17 Tailored (= anagram indicator) man’s suit (man’s suit = TSUNAMIS) produced huge waves (TSUNAMIS),
19 In a panic (SCARED) showing off (= anagram indicator) cedars (cedars = SCARED),
20 Centre (HUB-) near Cork (cork/lid = -CAP) (HUBCAP) or Trim for one of those doing a turn? (wheel trim = HUBCAP),
23 Turns up (= reversal indicator) all right (ok = -KO), caught (-C-), for example (e.g. = GE-), (GECKO) with that low life and doesn’t even blink! (GECKO).