Solution to 12D 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.
ACROSS
9 Game played by Beijing Airport security? (7,8) CHINESE CHECKERS [DD]
11 Secure line to hold uniform in washing (7) LAUNDRY {LA{U}ND}{RY}
14 Cook pasta or light snacks (5) TAPAS*
17 A bit of mushroom in pizza, say - not hot pepper (7) PIMENTO {PI{Mu...m}E}{~ThO} (Correction - {PI{Mu...m}E}{NOT*} - See comments)
21 Could be a lot closer to a lion's claw! (5) TALON {A+LOT}*{lioN}
23 Separate dirt from trifle (6,3) FILTER OUT {TRIFLE}* [RA]
25 Bottle thrower (7) PITCHER [DD]
30 Very small amount of what a scuba diver may want? (1,4,2,3,5) A DROP IN THE OCEAN [DD]
DOWN
1 Scheme conning average man, basically! (4) SCAM Acrostic &lit
2 Prejudice oddly seen in Brit also (4) BIAS {BrIt+AlSo}
3 Say, gondolier's cheerful supporter, per Spooner (8) FERRYMAN (~merry fan to ferryman)
5 Snack service in India's massage retreat (8) BHELPURI {HELP} in {I}{RUB}<=
7 Persuade courier to take book for son (4) BEND (-s+b)BEND
8 Cycle right outside parking spot (4) ESPY (-y}ES(+y)Y over {P}
12 Kilometre over a place's gone (5) K?P?T (Addendum - KAPUT {K}{A}{PUT} - See comments)
13 Dog biting master's tongue (5) TAMIL {TA{M}IL}
16 Writer entering empty set, exhausted (5) SPENT {Se{PEN}T}
18 Cancelled European band's subscription (8) OFFERING {OFF}{E}{RING}
20 Forget to supervise (8) OVERLOOK [DD]
22 Broken mobile phone stores nothing (2,4) NO HOPE {N{O}HOPE*}
26 Basically tournaments use really fresh grass (4) TURF Acrostic
28 Judgement of advocate initially taken down (4) IDEA (-a)IDE(+a)A
29 Nobody listened to sister (4) NONE (~nun)
Reference List
Minute = M, Line = RY, Uniform = U, University = U, Large = L, India = A, Sex = IT, Conservative = C, Queen = QU, Book = B, Son = S, Parking = P, Kilometer = K, Master = M, The in French = LA, European = E