Solution to 6D 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 Highest robbery of police car in Royal England? Perhaps yes (5,7) GRAND LARCENY {CAR+R+ENGLAND}*{Y} What's the role of Police? - See comments
9 Unwell mostly assumes to remain unemotional (5) STOIC {S{TO}ICk}
10 You old saint! Get ready for once (9) YESTERDAY {YE}{ST}{READY*} Anind?
11 Maybe cricket ball in climatic condition claims wicket for league (7) LEATHER (+l-w)LEATHER
12 See them in courts in groups chopping head (7) RACKETS bRACKETS
13 Viewers finally will become mad with pay channel (8) SPILLWAY {v...rS}{WILL+PAY}*
14 Slip of tongue initially in assurance (5) STEAL {S{To...e}EAL}
18 One is out of the one or the other number (5) ETHER EiTHER
20 Cannabis and cocaine hidden in belt. Money down (4,4) SPOT CASH {S{POT}{C}ASH}
24 Sweet liqueur at a fair deal (7) RATAFIA*
26 Drunken enemies beheaded by socialist in revenge (7) NEMESIS {eNEMIES*}{S}
27 Delivery may mostly lose energy forever (9) MATERNITY {MAy}{eTERNITY}
28 Foreign Secretary heartily leaves for good ally (5) ALIGN ALI(-e+g)GN
29 What a cabdriver might do to cheat? (4,3,1,4) TAKE FOR A RIDE [DD]
DOWN
1 Doing nothing? Eat bananas to reduce weight (2,2,1,4) GO ON A DIET {DOING+O+EAT}*
2 Maybe stout student enters in a high school without shoes initially (7) ALCOHOL {L} in {A}{sCHOOL*}
3 Daughter may dare to build castles in the air (8) DAYDREAM {D+MAY+DARE}*
4 Sailor's language almost meaningless (6) ABSURD {ABS}{URDu}
5 Work force lay-off - That is the style of Head! (4-3) CREW-CUT {CREW}{CUT}
6 New adult content, long for such magazine (5) N?D?E (Addendum NUDIE {N}{adUlt}{DIE} - See comments)
7 Unable to save (7) USELESS {USE LESS}
8 Traveller's coordinate found in the navigational aid by Yankee (5) GYPSY {G{Y}PS}{Y}
15 Desperate maiden saw water (5,4) ADAMS WINE*
17 Running after drink and pipe (7) CHASING {CHA}{SING}
19 Put the screws on workmen for celebrations inside (7) ENFORCE [T]
21 Kettledrums, originally Indian mridangams in inappropriate revolution (7) TIMPANI {In...n}{Mr...s} in {INAPT<=}
25 Ill-treat tropical fish (5) TETRA*
Reference List
Royal = R, Yes = Y, You old = YE, Saint = ST, Wicket = W, League = L, Cocaine = C, Socialist = S, Good = G, Student = L, Coordinate = Y, Yankee = Y, Pressure = P