Across:
1 Outlets (HO-SES) welcoming writing (from the 3Rs = -R-) (HORSE) by nags (HORSES),
4 Drawing the inference (ASSUMING) from new (= anagram indicator) mags in US (mags in US = ASSUMING),
9 Unfinished (t) shelter (‘tent’ without ‘t’ = TEN-) causes offence to religious type (sin) up (= reversal indicator) (sin = -NIS) (TENNIS) for the game (TENNIS),
10 Note (music note B-) the first sign of spring (Aries = -A-RIES) around Kildare (-KE-) (BAKERIES) in operations making plenty of dough (BAKERIES),
12 Most of those in 1 across (most of ‘horses’ = HORSE-) get away (shoo) we hear (= homophone indicator) (‘shoo’ = ‘shoe’ = -SHOE) (HORSESHOE) with lucky charm (HORSESHOE),
13 Is not in clues (‘is’ not ‘in clues’ = nclue = UNCLE) provided by the family (UNCLE),
14 Headhunters (TALENT SCOUTS) in 27 across (‘talent’ = TALENT) and 25 across (‘scouts’ = SCOUTS) (TALENT SCOUTS),
18 Addressing one issue (MAKING A POINT) working on The Spire, for example (MAKING A POINT),
21 Kind of manual (USERS manual) for draft-dodging fraudsters (‘draft’ dodging ‘fraudsters’ = users = USERS),
22 Prince Charming gets rid of cramp (‘Prince Charming’ gets rid of ‘cramp’ = rincehing = ENRICHING) by adding nutrients to diet, for example (ENRICHING),
24 Cockney (LONDONER) is hardly a people-person (LON-ER) protecting gangster (-DON-) (LONDONER),
25 Glaswegian (Scot = SCO-T-) takes in Universal (-U-) Studio primarily (‘Studio’ primarily = -S) (SCOUTS) with those who should be prepared (‘be prepared’ SCOUTS motto),
26 Spread (PERMEATE) the last of the ochre (the last of the ‘ochrE’ = -E-) in Tempera (Tempera = PERM-ATE) mix (= anagram indicator) (PERMEATE),
27 Genius (TALENT) produces fair amount of (half of) data (half of ‘daTA’ = TA-) for fast time (-LENT) (TALENT),
Down:
1 Toddies (hot shots = HOTSHOTS) produced by warm (HOT-), smashing (= anagram indicator) hosts (hosts = -SHOTS) (HOTSHOTS),
2 Hire (RENT) character (role) (RENT ROLL) on the radio (= homophone indicator) (‘role’ = ‘ROLL’) (RENT ROLL) in the pocket of the landlord (RENT ROLL),
3 Drains coming out of residential (‘drains’ coming out of ‘residential’ = eietl = ELITE) establishment (establishment/high society = ELITE),
5 Those spreading bad rumours (SCAREMONGERS) about (-RE-) penetrating fraud (scam = SCA-M-) starting off (c) with lots of fish (‘congers’ without ‘c’ = -ONGERS) (SCAREMONGERS),
6 The type of dump that can recycle everything (USEFUL TIP) offering a word to the wise (USEFUL TIP),
7 Showing signs of folly (IDIOCY) from endless (t) dope (‘idiot’ without ‘t’ = IDIO-) and drug? (cocaine = -C-) Yes! (-Y) (IDIOCY),
8 Get (get = G-ET) over question (-ASK-) (GASKET) from Seal (seal = GASKET),
11 Severe reprimand (CHASTISEMENT) is similar to 19 down (CHASTISE- similar to ‘pull up’) on Worker’s (-MEN-) Square (-T square) (CHASTISEMENT),
15 The Ancient Mariner’s (NOAH’S) port (Dover), for the most part (r) (‘Dover’ without ‘r’ = DOVE) (NOAH’S DOVE), offered an olive branch to survivor of the flood (NOAH’S DOVE),
16 Reading (from the 3Rs = R-) Euclid (Euclid = -ID-CULE) around island (-I-) (RIDICULE) leads to contempt (RIDICULE),
17 Moving (= anagram indicator) gates (gates = STAGE) to lay down (SET down) (STAGE SET) play area (STAGE SET),
19 Park (PULL UP) to stop one of those in 1 across (PULL UP ‘horses’ / stop one ‘horse’),
20 It’s more spiteful (MEANER) to get out of agreement (‘get’ out of ‘agreement’ = aremen = MEANER),
23 Has no input in occupation (has no ‘input’ in ‘occupation’ = occao = COCOA) producing hot drink (COCOA).