Across:
1 It’s a mystery (RIDDLE) what this is? (this clue is a RIDDLE),
4 Begun old (begun old = BLUDGEON) novel (= anagram indicator) in club (BLUDGEON),
9 Is not working (ABSENT) for graduate (BA) returning (= reversal indicator) (BA = AB-) to Capital (capital/money = ‘cent’) Radio? (= homophone indicator) (‘cent’ = ‘sent’ = -SENT) (ABSENT),
10 Organisation (UN-) is being wound up perhaps (-ENDING) (UNENDING) and it’s taking an age (UNENDING),
12 China gets (China gets = TEACHINGS) lost (= anagram indicator) in doctrine (TEACHINGS),
13 Deliberately gets rid of Bradley (‘deliberately’ gets rid of ‘Bradley’ = eliet = ELITE) from select group (ELITE),
14 Church (-CE-) is consumed by meeting in Parliament (SE-SSION-), leading the pack (first = 1ST = -IST) (SECESSIONIST) advocating a Hard Brexit perhaps (SECESSIONIST),
18 A French- (a = UN-)-American (-A-) cold (-C-) mushroom (-CEP-) meal (-TABLE) (UNACCEPTABLE) is not on (UNACCEPTABLE),
21 Set off from near east (‘set’ off from ‘near east’ = neara = ARENA) for the sports stadium (ARENA),
22 Make the decision to put boots on the ground perhaps (PICK A TEAM) to get dig in (PICK) at elite unit (A-Team = A TEAM) (PICK A TEAM),
24 Papers (I-D) covering Pinter (Pinter = -NTREPI-) play (= anagram indicator) (INTREPID) describing style of No Shrinking Violet? (INTREPID),
25 Trim (r) golf club (‘driver’ without ‘r’ = DRIVE-) is not the first (‘Not’ the first = -N) (DRIVEN) to be motivated (DRIVEN),
26 The Loire (the Loire = HOTELIER) cook (= anagram indicator) works in the hospitality business (HOTELIER),
27 The Blades (SWORDS) could have been discovered in Dublin (SWORDS).
Down:
1 It’s an indication of how good the coach is working (ROAD TEST), by the way (ROAD), at trial (TEST) (ROAD TEST),
2 The latest intelligence (DESPATCH) of squabble (-SPAT-) between Germany (Germany domain = .de = DE-) and Switzerland (-CH) (DESPATCH),
3 Call the tune dismissing Attlee (‘call the tune’ dismissing ‘Attlee’ = clhun = LUNCH) making a meal of it (LUNCH),
5 What those going to the guillotine didn’t get presumably (LONG SENTENCE) in life? (LONG SENTENCE),
6 Watering hole (pub = -INN-) surrounded by wild (= anagram indicator) desert (desert = D-ER SET) (DINNER SET) is used daily by those in service (DINNER SET),
7 Turns up (= reversal indicator) bug (rile = EL-IR) catches one (-I-) mystery man (-X-) (ELIXIR) offering magic potion (ELIXIR),
8 Return (= reversal indicator) piece (gun = NUG-) to secure (-GET) (NUGGET) bit of gold (NUGGET),
11 Reason for teaching someone a lesson (INDISCIPLINE) provided it’s popular (IN-) field of study (-DISCIPLINE) (INDISCIPLINE),
15 Look for oil (SINK A WELL) in ruin (SINK) approaching start of (‘Approaching’ start of = A) spring (WELL) (SINK A WELL),
16 Local hooligan (yob) loses the head (y) (‘yob’ without ‘y’ = OB-) with one of those in court (-SERVER on tennis court) (OBSERVER) – presumably, it’s the witness? (OBSERVER),
17 Faxed (sent = S-ENT-) over letters for extraordinary meeting (Extraordinary General Meeting = -EGM-) for school (-S) (SEGMENTS) divisions (SEGMENTS),
19 Loud (GARISH) head barman (Attorney General = AG = GA-) knocks back (= reversal indicator) most of (i) the local whiskey (Irish) (‘Irish’ without ‘i’ = -RISH) (GARISH),
20 Take a hike (take a hike = BEAT IT) in casual clobber (BEAT) that goes with one (I-) shirt (-T shirt) (BEAT IT),
23 It’s a sign (ARROW) of what’s raw or (raw or = ARROW) off (= anagram indicator).