Across:
1 Review (= reversal indicator) had (had = DAH-) fairly (half of) lied (half of ‘lied’ = -LI-) about introduction (‘about’ introduction = -A) (DAHLIA) by Mexican type in the plot (DAHLIA),
4 Simmering (BOIL DOWN) rage (BOIL) in part of Ireland? (Co DOWN) (BOIL DOWN),
9 A Milan (a Milan = ANIMAL) butcher (= anagram indicator) provides food for the meat eaters (ANIMAL),
10 Charge (STAMPEDE) foreign politician (-MP-) in seated (seated = STA-EDE) building (= anagram indicator) (STAMPEDE),
12 All set (READY TO GO) for Keane (“Keane” = “keen”) reportedly (= homophone) to retire (keen to retire = READY TO GO),
13 Some Winnipeg dunderhead (some ‘Winnipeg dunderhead’ = egdun) upset (= reversal indicator) (egdun = NUDGE) by dig in the ribs (NUDGE),
14 You’ll need this get through 20 down, for instance? (MOUNTAIN ROAD to get through ‘Sierra’) That’s a pass! (mountain pass = MOUNTAIN ROAD),
18 Travel agent perhaps (HOLIDAYMAKER) is taking a break (HOLIDAYMAKER),
21 Leave port (sail = -LIAS) for a (A-) Rising (= reversal indicator) (ALIAS) with an assumed name (ALIAS),
22 Crisis (EMERGENCY) for most of (n) the workers (‘men’ without ‘n’ = -ME-) in New (= anagram indicator) Regency (Regency = E-RGENCY) (EMERGENCY),
24 Two pubs (bar + bar = BARBAR-) for one (-I-) starting college (starting ‘college’ = -C) (BARBARIC) – it’s savage! (BARBARIC),
25 Greats from East Germanic (‘Greats’ from ‘East Germanic’ = emanic = CINEMA) film house (CINEMA),
26 Look for employees to work (HEADHUNT) on top of the house (top of the ‘house’ = H-), possibly (= anagram indicator) haunted (haunted = -EADHUNT) (HEADHUNT),
27 Interfere (MEDDLE) with State’s (states = homophone indicator) gold or silver (gold or silver “medal” = “MEDDLE”).
Down
1 In retrospect (= reversal indicator), small (-S) march (mar = -RAM-) is great (-G-) for backing (backing/aid = DIA-) (DIAGRAMS) representations (DIAGRAMS),
2 Toast (HAIL) sailor (able-bodied seaman = A B-) from America (-US) (HAIL A BUS) with flag down? (HAIL A BUS),
3 Carted off dairy cattle (‘carted’ off ‘dairy cattle’ = aiytl = ITALY) over the Swiss border (ITALY),
5 Old (O-) Tudor (Tudor = -UTDO-R) compound (= anagram indicator) around Oxford (-O-) with pastimes (GAMES) (OUTDOOR GAMES) of jousting and archery (OUTDOOR GAMES),
6 Melon (melon = LEMON) salad (= anagram indicator) and lamb (lamb = BALM) stew (= anagram indicator) (LEMON BALM) with fragrant herb (LEMON BALM),
7 Put too much emphasis on (OVERDO) removing elms out of velodromes (out of ‘elms’ from ‘velodromes’ = vodroe = OVERDO),
8 Selection drops lot (‘selection’ drops ‘lot’ = seecin = NIECES) of family members (NIECES),
11 Optimistic reaction (POSITIVE) to The Omen (SIGN) (POSITIVE SIGN) is a plus (POSITIVE SIGN),
15 Leader of country (TAOISEACH) to share knowledge (share knowledge/teach = T-EACH) about county (Laois) with no leader (l) (‘Laois’ without ‘l’ = -AOIS-) (TAOISEACH),
16 Ruffian (SKINHEAD) is depressed (S-AD) over family (-KIN-) on heroin (-H-) and ecstasy (-E-) (SKINHEAD),
17 Private place (ARMY BASE) for major employer (ARMY BASE),
19 Awful (bad) short (d) (‘bad’ without ‘d’ = -BA-) in capital (CAS-H) (CASBAH) with citadel (CASBAH),
20 Is rare (is rare = SIERRA) rock (= anagram indicator) from a range of mountains? (SIERRA),
23 Streaming audio without animator’s (‘streaming audio’ without ‘animators’ = egudi = GUIDE) pilot (GUIDE).