Across:
1 Help out (MUCK IN) presenter (emcee = MC = M-C-) entertaining Ulster (-U-) family (kin = -K IN) (MUCK IN),
4 Facial hair (WHISKERS) found at the end of 18 across (found at end of ‘cat’s whiskers’ = WHISKERS),
9 Mystery man (X-) and (AN-D-) a (-A-) trespasser from university (-U-) (XANADU) in idyllic place (XANADU),
10 Unfortunate (= anagram indicator) promise (promise = IMPOS-ER) over time (-T-) (IMPOSTER) leads to fraud (IMPOSTER),
12 Starter of 14 across (starter of ‘marmalade cat’ = MARMALADE) with spread (MARMALADE),
13 Man (George) dropping ecstasy (e) (‘George’ without ‘e’ = GORGE) in The Canyon (GORGE),
14 Preserve (MARMALADE) most of those opening 18 across (most of those opening ‘cats whiskers’= most of ‘cats’ = CAT) (MARMALADE CAT) for pet who has the look of Ginger? (MARMALADE CAT also called ginger cat),
18 Provides the canapés and champagne (caters = CAT- -ERS) welcoming school (-S) – the one employed in the kitchen (-WHISK-) (CAT’S WHISKERS) by the receivers (crystal radio receivers = CAT’S WHISKERS),
21 Opens 6 down (opens ‘shot glass’ = SHOT-) with primarily swings (primarily ‘Swings’ = -S) (SHOTS) and slides (SHOTS),
22 Recalls (REMEMBERS) it’s about (RE-) those in society (-MEMBERS) (REMEMBERS),
24 Give up (PACK IT IN) stuff inside suitcase (PACK IT IN),
25 Cope (MANAGE) with East German not getting rest (‘East German’ not getting/without ‘rest’ = eagman = MANAGE),
26 Ripped to shreds (TATTERED) in most of (s) the rags (tatters) (‘tatters’ without ‘s’ = TATTER-) by journalist (-ED) (TATTERED),
27 Most affected, quaint (TWEEST) tenor (T-) with ultimate voice (ultimate ‘voicE’ = -E-) in Where the Sun Sets (west = -WE-ST) (TWEEST).
Down:
1 The most you'll get (MAXIMUMS) from graduate (MA-) with eleven (-XI-) family members (-MUMS) (MAXIMUMS),
2 Opposed to (CONTRARY to) criminal (CON-) trial (TR-Y) involving a (-A-) third party (third ‘paRty’ = -R-) (CONTRARY),
3 Support (aid = -DIA) for Ulster (NI = IN-) goes all around (= reverse all indicator) (INDIA) the country (INDIA),
5 Stressed (HAMMERED HOME) and drunk in the local (‘drunk’ in the local sense = HAMMERED), getting the digs (HOME) (HAMMERED HOME),
6 Photograph (SHOT) legend (‘leG’ end = G-) with young woman (-LASS) (SHOT GLASS) holding a little tipple of whiskey, for one (SHOT GLASS),
7 Diner (EATERY) from steelyard drops LSD (‘steelyard’ drops ‘LSD’ = teeyar = EATERY),
8 Guard (SCREEN) detected (seen = S-EEN) getting hold of credit (-CR-) (SCREEN),
11 Serving of lunch on a boat presumably (NAVAL SERVICE) for that crafty lot defending the coast (NAVAL SERVICE),
15 Water (ADAM’S WINE) and top (‘And’ top = A-) crackers (mad = -DAM-) served up (= reverse indicator) to local unpleasant person (swine = -S WINE) (ADAM’S WINE),
16 Model (= anagram indicator) elated (elated = DEL-ATE) over, for example (e.g. = -EG-), (DELEGATE), commission (DELEGATE),
17 In the beginning (‘In’ the beginning = I) taken in by easiest (easiest) problem (= anagram indicator) (easiest = AS SEE IT) (AS I SEE IT), in my view (AS I SEE IT),
19 Heading off (b) for celebration (bash) (‘bash’ without ‘b’ = ASH) around (= reverse indicator) crest (tip = PIT) (ASH PIT) – the setting for the last bit of The Inferno (ASH PIT),
20 In retrospect (= reversal indicator), a fair amount of (half of) memo (half of ‘ meMO’ = mo = -OM) covered by diplomacy (tact = tcat = T- CAT) (TOMCAT) relates to one of those on the prowl (TOMCAT),
23 A cry from 14 across (MIAOW from ‘marmalade cat’) for women (-W) having nothing (-O-) to do with objective (aim) volte-face (= reversal indicator) (aim = MIA-) (MIAOW).