Solution to 16D 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 Roadwork Company clerk at the outskirts to get a valve, say (8) STOPCOCK {ST}{OP}{CO}{ClerK}
5 At the same time as wife, husband is crossing line on time (6) WHILST {W}{H}{IS}{L}{T}
10 Rowdy's cover at slums exposed (7) HOODLUM {HOOD}{sLUMs}
11 A plane curve can be seen at eclipse, after initial cover disappears left (7) ELLIPSE {E(-c+l)LLIPSE}
14 Doctor reins her fear with an aerosol spray, perhaps (3,9) AIR FRESHENER*
18 Happiness at party after Sita's mingling (12) SATISFACTION {FACTION}<=>{SITAS}*
21 Box kept near gate has arrack, essentially for the band (9) ORCHESTRA {CHEST}<=>{OR} and {arRAck}
25 In release, Tom said - process of the temporal bone (7) MASTOID*
26 Break off with blonde, sister relayed (6) DESIST [T]
27 Deadly deli, i.e barmen at both ends menacing (8) INEDIBLE {DELI+IE+Ba..eN}
DOWN
1 Rift in school is mushrooming primarily (6) SCHISM {SCH}{IS}{Mu...g}
2 Aromas are forbidding in old society (6) ODOURS {O}{DOUR}{S}
3 Disconcerted masculine insults (9) CALUMNIES*
4 Quotient of two integers derived from standard operation using Faraday's rule (6,8) COMMON FRACTION {COMMON}{F}{R}{ACTION}
6 Curse over 50 individuals makes them a genus of snail (5) HELIX {HE{L}{1}X}
7 A connector in friend's back's jagged at the tip, points to erosion at both ends (3,5) LAP JOINT {PAL<=}{Ja...d}{pOINTs}
9 Our primitive ancestor learnt, had men adopt one development (11,3) NEANDERTHAL MAN {LEARNT+HAD+MEN}* over {AN} As per the clue AN is also part of anagram fodder, making it indirect!
15 Caught son turning inside on the description of a facial feature (4-5) HOOK-NOSED {HOOK{SON<=}ED}
16 Different fool retweeted without no end (8) ?S?O?T?D ()
17 Holders assemble rocket in ship (8) STOCKERS {S{ROCKET*}S}
19 One pill at the beginning and end by doctor's order advances calmness (6) APLOMB {A}{PilL} and {MB}<=>{O}
20 Vehicle found in a small ridge (6) SLEDGE {S}{LEDGE}
22 Time lost in matches for betting odds (5) EVENS EVENtS
Reference List
Road = ST, Work = OP, Company = CO, Wife = W, Husband = H, Line = L, Time = T, Left = L, Second = S, Old boy = OB, Gate = OR, North east = NE, School = SCH, Old = O, Society = S, Faraday = F, Rule = R, 50 = L, Individuals = I, Ship = SS, Order = O, Small = S
