Across:
1 Obtain information on computers (ACCESS) from 100 (Roman 100 = -C-) in criminal (= anagram indicator) cases (cases = AC-ESS) (ACCESS),
4 Raises my (raises my = EMISSARY) misprint (= anagram indicator) in The Messenger (EMISSARY),
9 Old city (-UR-) bishop (-B-) taken in by shady type (T-AN) (TURBAN) covering the head (TURBAN),
10 Rough calculation (ESTIMATE) for leader of Easter (leader of ‘Easter’ = E-) Rising (= reverse indicator) – it’s (its = -STI-) a short check for those on the board (short ‘checkmate’ on chess board = mate = -MATE) (ESTIMATE),
12 Traveller’s guide (M-AP) covering where in France (‘where’ in France = -OU-) close to Paris (close to ‘pariS’ = -S-), station (RTE) sets up (= reverse indicator) (RTE = -ETR-) (MOUSETRAP) device for capturing vermin (MOUSETRAP),
13 Lubricated (OILED) agent is not part of delegation (‘agent’ is not part of ‘ delegation’ = delio = OILED),
14 American (A) cons (fools = FOOL’S) taken to task (ERRAND) (A FOOL’S ERRAND) for sending the newcomer for the bubble in the spirit level (A FOOL’S ERRAND),
18 To reign (RULE THE ROOST) in Russia (RU-), allow (let = -LE T-) victor (victor = hero = -HE RO-) to take post (post), getting rid of the leader (p) (‘post’ without ‘l’ = -OST) (RULE THE ROOST),
21 The House (Dáil = -LIAD) initiative, primarily (‘Initiative’ primarily = I-) brings up (= reverse indicator) (ILIAD) an earlier work that’s all Greek to me (ILIAD),
22 Pay no attention to (IGNOR-E) introductions to a new chapter (introductions to ‘A New Chapter’ = -ANC-) inside (IGNORANCE) Lack of Intelligence (IGNORANCE),
24 Gun dog (retriever) with no tail (r) (‘retriever’ without ‘r’ = RETRIEVE) gets to fetch (RETRIEVE),
25 It gets thrown about (DISCUS) in endless (s) debate (‘discuss’ without ‘s’ = DISCUS),
26 Those pointing fingers at other types (ACCUSERS) of coppers (CUs = -CUS-) hiding in shrubs (AC-ERS) (ACCUSERS),
27 Fresh (UNUSED) mint? (UNUSED).
Down:
1 Upstart (‘Up’ start = -U-) in Tuam (Tuam) crazy (= anagram indicator) (tuam = A- TUM-) with Nationalist (-N-) Revolutionary (= reversal indicator) foreign article (la = -AL) (AUTUMNAL) from the time of the fall in New England! (AUTUMNAL),
2 Material (CORDUROY) is cut for delivery (umbilical CORD-) to your (your) building (= anagram indicator) (your = -UROY) (CORDUROY),
3 Remove Vandyke (SHAVE Vandyke beard) relic from chevaliers (‘relic’ from ‘chevaliers’ = hevas = SHAVE),
5 Address (miss) brief (s) (‘miss’ without ‘s’ = MIS-) put in jail (-APPREHEND) (MISAPPREHEND) by mistake (MISAPPREHEND),
6 His (his) adaptation (= anagram indicator) (his = SHI-) might (power = -POW-ER) cover point (North = -N-) (SHIPOWNER) produced by one employing his craft overseas (SHIPOWNER),
7 Biblical story (parable) that needs no introduction (p) (‘parable’ without ‘p’ = ARABLE) is tailor-made for The Ploughing (ARABLE),
8 Throws in the towel (YIELDS) and tediously misses out (‘tediously’ misses ‘out’ = edisly = YIELDS)
11 One who never questions the principles (TRUE BELIEVER) of what’s in heads of teal, robins, uncommon eiders (heads of ‘Teal Robins Uncommon Eiders’ = TRUE) and swallow local (‘swallow’ local = BELIEVE-) to river (-R) (TRUE BELIEVER),
15 Makes up stories (lies = L-IES) about unusual (= anagram indicator) Ad Run (ad run = -AUNDR-) (LAUNDRIES) produced by those who take credit for everything coming out in the wash (LAUNDRIES),
16 Draws a parallel with (CONNECTS) fool (CON-) caught (-C-) in fishing gear (nets = -NE-TS) (CONNECTS),
17 Drew attention (STRESSED) serving up (= reverse indicator) a number of jelly and ice creams, for example (desserts = STRESSED),
19 The range (SIERRA) of some folksier rapper (some ‘folkSIER RApper = SIERRA),
20 Strange (= anagram indicator) tactic (tactic = TIC-TAC) used by accountant, of course (TIC-TAC / semaphore used by turf accountants on racecourse)
23 Hold sway (REIGN) like in 18 across (REIGN is like ‘rule the roost).