Across:
1 Something for the postman to deliver (PACKET) working for the post office overseas perhaps? (mail boat = PACKET boat),
4 Anonymous (A-) police department (Criminal Investigation Department = -CID) experiment (TEST) (ACID TEST) will conclusively tell you everything you need to know (ACID TEST),
9 Start of trouble (start of ‘Trouble’ = T-) close to Doha (close/end to ‘dohA’ = A) in country on the Arabian Peninsula (Oman = -O MAN) (TO A MAN) and that’s without exception (TO A MAN),
10 Crucial (crucial) play (= anagram indicator) (crucial = CIRCULA-) with right (-R) (CIRCULAR) message for wider distribution (CIRCULAR),
12 Turns up (= reverse indicator), fire (fire = -ERIF) on ship (SS = -S S-) starts on newer and (starts on ‘Newer And’ = na = -AN-) bottom decks (bottom ‘deckS’ = S) (SANS SERIF) – the straightforward type that appeals to writers (SANS SERIF type/font),
13 Confined in a way (CAGED) in decorating without nitro? (‘decorating’ without ‘nitro’ = decag = CAGED),
14 This is so obvious (DEAD GIVEAWAY) to Will presumably? (a will = DEAD GIVEAWAY),
18 A little drunk (MERRY-) from Ring (-O) consumed by track (ground = G-ROUND) (MERRY-GO-ROUND) by Fairground Attraction (MERRY-GO-ROUND)
21 Heard (= homophone indicator) course (‘course’ = ‘coarse’ = HARSH) is rough enough (HARSH),
22 Exchange things (barter = BAR-TER) about teacher (Sir) retiring (= reverse indicator) (Sir = -RIS-) (BARRISTER), in brief locally (local ‘brief’ = BARRISTER),
24 Popular (IN-) open-top (open ‘Top’ = -T-) racy (-RUDE-) car – ultimately (‘caR’ ultimately = -R) (INTRUDER) produced by one of those breaking into the business (INTRUDER),
25 Capsized (= reverse indicator) brothers (bros = -SORB) from dinghy start (‘Dinghy’ start = -D-) to head ashore (head ‘Ashore’ = A-) (ADSORB) and gather together on the surface (ADSORB),
26 Is (-IS-) found in March (march/parade = PARAD-E) (PARADISE) in a place of utter bliss (PARADISE),
27 Heals (heals) sick (= anagram indicator) (heals = HAS-LE) around school (-S-) (HASSLE) with pain in the neck? (HASSLE).
Down:
1 Update is (update is) signal (= anagram indicator) (update is = PUT ASIDE) to save for later (PUT ASIDE),
2 Series (CHAIN-) was (was) reviewed (= reversal indicator) (was = -SAW) (CHAINSAW) by one capable of cutting stuff down to size (CHAINSAW),
3 Tests (EXAMS) part of complex Amstrad (part of ‘complEX AMStrad’ = EXAMS),
5 Bluff (bluff/ridge = CLIFF-) those employed by the dry cleaners (-HANGERS) (CLIFFHANGERS) keeping everyone in suspense (CLIFFHANGERS),
6 What one of the beach bums might take (DECK CHAIR) from Grace (grace/adorn = DECK) Chesterfield? (Chesterfield sofa/seat = CHAIR) (DECK CHAIR),
7 Grave speech (EULOGY) by Leo, Guy (leo guy), Rocky (= anagram indicator) (leo guy = EULOGY),
8 Lecture (TIRADE) produced by institute (-I-) in that line of work (T-RADE) (TIRADE),
11 It’s very serious (GRAVE) for shoplifters (ROBBERS) (GRAVE ROBBERS) and those on the take, of late (GRAVE ROBBERS),
15 Bound to go to the dogs (GREYHOUND racing) with American coach (GREYHOUND bus/coach)
16 For those doing the bidding (AUCTIONS) of university (-U-) consumed by lawsuits (actions = A-CTIONS) (AUCTIONS),
17 Skilled at (A-BLE) hiding what’s in Pandora’s (what’s in ‘panDORAs’ = -DORA-) (ADORABLE) chocolate box! (ADORABLE),
19 Fishy type (SHRIMP) of sympathiser? None as yet! (‘sympathiser’ none/without ‘as yet’ = mphisr = SHRIMP).
20 Women (W-) get confirmation (religious rite/ confirmation = -RITE-) by writing (one of 3Rs = -R) (WRITER) to blogger (WRITER),
23 Support (aid = -DIA) for Ulster (NI = IN-) up (= reverse indicator) (INDIA) country (INDIA).