Solution to 6A has been deliberately left unsolved and is to be answered only by a non-regular / novice commenter, with proper annotation. Those who have answered earlier in the week, please give others a chance.
Open for anyone to answer, if not solved by 1 PM.
ACROSS
1 Vehicle reverses on street at a circle, basically near a small powerhouse (10) SUBSTATION {BUS<=}{ST}{AT}{1}{O}{Near}
6 Girl at a US city takes a ship (4) ?A?S (Addendum - LASS {LA}{SS} - See comments)
10 Registration made with specialist for admission of rabbi and aged people without 500 (9) ENROLMENT {EN{R}{OLd MEN}T}
11 Mark found in Insta intermittently (5) STAIN [T]
15 Let off former lover, one using speed (9) EXONERATE {EX}{ONE}{RATE}
17 Thracian gladiator to wrestle with wild cat at America (9) SPARTACUS {SPAR}{CAT*}{US}
21 Bring the foot down slowly, tread on mat, prostrate in the front (5) STOMP Acrostic
22 Aunt roasting some lamb, gets burned leading to suffocation (13) STRANGULATION {AUNT+ROASTING+Lamb}*
24 A German had a gemstone (5) AGATE {A}{G}{ATE}
25 Smoking nicotine around hospital puts one precariously close to trouble (2,4,3) ON THIN ICE {NICOTINE}* over {H}
26 A piece of satin or same type of fabric (4) SILK {Sa..n}{ILK}
27 Persistent and aggressive sellers hound a specialist (10) RELENTLESS {REL{ENT}LESS*}
DOWN
1 Ice mixed with some cocaine? PT miss has doubt (10) SCEPTICISM {ICE+C+PT+MISS}*
2 Predatory man seen in pub, Dracula lacking love, is drunk (9) BARRACUDA {BAR}{DRACUlA}*
4 Article on a tree abridged at the stage (7) THEATRE {THE}{A}{TREe}
5 One then got tails cut working constantly (2,3,2) ON THE GO {ONe}{THEn}{GOt}
7 Observant king replaced by another and nothing changes (5) AWARE AWA(-r+k}E
8 Chant for good after lapse (4) SING {G}<=>{SIN}
9 As an animal loses the first step on land... (6) ASHORE {AS}{HORsE}
13 ...unarmed Pole was confused and received a few directions (10) WEAPONLESS {POLE+WAS}*over {N}{E}{S}
16 Heartless. Hate police officer in England; original inhabitant (9) ABORIGINE {ABhOR}{IG}{IN}{E}
18 Lathe operator in a bad pitch (6) TURNER [DD]
19 Hint at criminal's message (7) CONNOTE {CON}{NOTE}
20 Travel stop, having tea, in conversation with the French (7) SHUTTLE {SHUT}{(~tea)T}{LE}
21 Triages conducted at island, perhaps (7) SEAGIRT*
22 A sluggish person uncovers cycling (5) SNAIL (+s)SNAIL(-s)
23 Service the crowd (4) MASS [DD]
Reference List
Street = ST, Ship = SS, Specialist = ENT, Rabbi = R, 500 = D, Corps = C, Left = L, German = G, Hospital = H, Cocaine = C, Love = L, Germany = G, King = K,R, Good = G, England = E, The in French = LE