Solution to 7D 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 No diet, pie baked, consumed ten in a trip (10) EXPEDITION {NO+DIET+PIE}* over {X}
6 Headless chicken? No, some other birds (4) OWLS fOWLS
10 Keep spirit in front of hearth (9) FIREPLACE {PLACE}<=>{FIRE}
11 Opposition leader managed to squander good agency (5) ORGAN {Op...n}{R{G}AN}
12 Crowd caught ex-con ultimately with grip, but some cash missing (13) CONCENTRATION {C}{ONCE}{eoN}{TRAcTION}
14 Be back at base (4) EVIL<=
15 Closeness seen with female at new diner, small joint perhaps (10) FRIENDSHIP {F}{DINER*}{S}{HIP}
17 Say, killers' target? Ice innovatively with sulphur (10) CIGARETTES {TARGET+ICE}*{S}
21 Toppers in Oxford really ace law exam (4) ORAL Acrostic
23 Supportive of lower experience (13) UNDERSTANDING {UNDER}{STANDING}
25 Stick with previous wife ultimately (5) PASTE {PAST}{wifE}
26 Marines touring California are perhaps Californians (9) AMERICANS {AMERI{CA}NS*}
27 Cannot erase hidden message (4) NOTE [T]
28 Following order and rule, collect papers in the latter half from 26, say (10) FOREIGNERS {F}{O}{REIGN}{papERS}
DOWN
1 Icy niece working to overtake two females with skill (10) EFFICIENCY {ICY+NIECE}* over {FF}
2 Father let in girl for training (9) PARENTING {PA}{RENT}{IN}{G}
3 Went down with investigator and public prosecutor on preliminary enquiry with director (6) DIPPED {DI}{PP}{En...y}{D}
4 Vehicle found at lot near gate (7) TRACTOR {TRACT}{OR}
5 One does this in theatre at work time, time before excision primarily (7) OPERATE {OP}{ERA}{T}{Ex...n}
7 Try hacking network, buggy perhaps (5) W?G?N (Addendum - WAGON {WA{GO}N} - See comments)
8 Gentleman from Kolkata gets director post (4) SEND {SEN}{D}
9 Policeman gets a bomb reproduced (6) COPIED {COP}{IED}
13 A butcher loses pig over nothing and asks for forgiveness (10) APOLOGISES {A}{LOSES+PIG}* over {O}
16 Speed up, nearly taking staff across island in cyclone (9) HURRICANE {HURRy}{CANE} over {I}
18 Hills are cooler, perhaps, heading off close to plains (6) RIDGES {fRIDGE}{p...nS}
19 Ran amok in commotion of storm (7) TORNADO {TO{RAN*}DO}
20 To meet ex-Romeo travels faraway (7) EXTREME {MEET+EX+R}*
22 Friend in Gurgaon crushed by termination (6) ENDING [T]
23 Agitation at state settled (5) UPSET {UP}{SET}
24 Wide ball held near fence (4) OPEN {O}{PEN}
Reference List
Good = G, Caught = C, Female = F, Small = S, California = CA, Following = F, Order = O, Girl = G, Investigator = DI, Public prosecutor = PP, Director = D, Gate = OR, Work = OP, Time = T, Bomb = IED, Island = I, Commotion = TODO, Romeo = R