ACROSS1 Investigator probes organised crime using evidence (7) EMPIRIC - PI inside (crime)*
5 Fire getting cold and greyer (7) C,ASHIER
9 Accelerate in front of athlete who moves arms (9) GUN,RUNNER
10 Mass of rock crystal usually keeps hidden (5) TALUS - incl.
11 Old name for soldiers sometimes showing character (5) dd - THE Territorial Army
12 Maybe broody old dignitary, a sour drunk (9) O,VIP,A,ROUS - (sour)*
13 Person who's decorated not one ring of gold (5,8) NO,BEL L,AUREATE
14 Rating new filling dessert, able to take a lot in (9) AB,SORBE(N)T
15 Lack of balance provoked two lots of my tears (9) (my my tears)*
21 Weapon the German clutches doing wrong (9) D,ERRING,ER
24 Counted the minutes to retreat or resign (5) TIMED rev.
25 Juvenile article in Paris Match is a scoop (5) LAD,LE
26 Sample by writer, one who's not very good (9) POE,TASTER
27 Sort of material from one who's ineffectual and uninteresting (4-3) DRIP-DRY
28 Cry to encourage friend in hot pants (5-2) TALLY-HO - ALLY inside (hot)*
DOWN
1 Top important figure (6) (w)EIGHTY
2 Writer brought back offended, eponymous hero (9) PEN,DENNIS - SINNED rev.
3 Regret vocal jibe in stage whisper (7) RHUBARB - homophone for 'rue' + BARB
4 Petted dog, holding head (9) CA,NOODLE,D - dog = cad, bad man
5 Animation about heraldic colour that's a palace favourite (5) CORGI - CGI outside OR, which is the heraldic (French) term for 'gold'
6 Trouble is after having favourable prognosis (3,4) SET FAIR (is after)*
7 Northern home's inviting empty room (5) IGLOO - I(gnorin)G + LOO
8 I'm not sure about feminist opponent (8) RE,SISTER
17 Player given hand aligns with person on the right film (4,4,5) WEST SIDE S,TORY - west is a player given a hand in contract bridge
16 Son moved awkwardly, restrained (8) S,WADDLED
18 Pay bonus money through post (7) S,TIP,END
19 It's weird not having a partner round (7) ODDBALL - ODD (by itself) + BALL (round)
20 Amount of volume by orchestra's principal music player (6) STERE,O
22 Mostly, broadcaster and I spoke and spoke? (5) RADII - spokes in a wheel - RADI(o) + I
23 Fish very quietly bound by rope (5) GU,PP,Y