Solution to 18A 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
7 Linger outside park leading one animal (6) WAPITI {WA{P}IT}{1}
9 Diplomat arranging mass abroad (10) AMBASSADOR*
10 Voice is regularly suppressed in fights (4) VIES {VoIcE+iS}
11 Plan of regime finally ousting Welsh in new reign possibly (8) ENGINEER {r...mE} in {NEw+REIGN}*
12 Sweet-sounding music for two including originally lovely contralto (6) DULCET {DU{Lo...y}{Co...o}ET}
13 One in French hotel caught with director actively stopping reel continuously (5,3,5) ROUND THE CLOCK {UN}{HOTEL+C+D}* in {ROCK}
16 Minister from new circle (6) CLERIC*
18 Reverend's double course rejected (5,3) R?L?D ?U?
21 Delay report covering lecture essentially (4) STAY {S{lecTure}AY}
22 Rag this ordinary garden parties perhaps (10) PEDESTRIAN {PEDESTRIAN+RAG}*=GARDEN PARTIES [CA]
23 Old relative in favour of European support (8) FOREBEAR {FOR}{E}{BEAR}
24 Upset Republican bumping boxer's head in combat (6) RATTLE (+r-b)RATTLE
DOWN
4 Irregular distribution of aid blocking yields in return (8) SPORADIC {AID}* in {CROPS<=}
5 Tour starts to take relatives around various enjoyable locations (6) TRAVEL Acrostic
6 King, born idiot giving reaction without thinking (4-4) KNEE-JERK {K}{NEE}{JERK}
8 Without being detected abused a thread run about the German (5,3,5) UNDER THE RADAR {A+THREAD+RUN}* over {DER}
13 Maybe sister in joy backing brother in the end (8) RELATION {ELATION}<=>{b...eR}
14 Crack and lit heroin piece inhaled by doctor (8) DECIPHER {H+PIECE}* in {DR}
15 Tough to persuade bud sadly controlled by old judge (8) OBDURATE {O}{BUD*}{RATE}
17 Verses in religious education listing chants ignoring number (6) RHYMES {RE} over {HYMnS} How does E go in?
19 Again chase passion and shine (6) LUSTRE {RE}<=>{LUST}
20 Working in underpass initially dark (6) USABLE {Un...s}{SABLE}
Reference List
Park = P, Unionist = U, International = I, Welsh = W, One in French = UN, Caught = C, Director = D, European = E, Republican = R, Son = S, Note = TI, King = K, Born = NEE, The in German = DER, Heroin = H, Old = O, Religious education = RE, Number = N, Again = RE