No 13064, Monday 12 Oct 2020, Dr. X,

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.

ACROSS
1   Wager ruins Spooner causing problems (8) SETBACKS (~ bet sacks)
5   Abuses guys chasing girl (6) GROPES {G}{ROPES}
10 Reject salesperson getting a little lazy during work (7) REPULSE {REP}{U{Lazy}SE}
11 Orderly in train catches rebellious lout (7) REGULAR {RE{LUG<=}AR}
12 Wail about tackling new relative after marriage (2-3) IN LAW {I{N}LAW*}
13 Old boy babbles about nursing a little silly infatuation (9) OBSESSION {OB}{NOI{Si..y}SES<=}
14 Endure criminal acquiring firm unopposed (12) UNRESTRICTED {UNRE{STRICT}ED*}
18 Tired associate leaves to snort speed at bar in Columbia, say (5,7) SPACE SHUTTLE  {S{PACE}{SHUT}TaLE}

21 Secures top place finally in decisive victory (9) LANDSLIDE {LANDS}{LID}{p..cE}
23 Excellent proposal to restrain Republican (5) BRILL {B{R}ILL}
24 Soldiers in depot assembled missile (7) TORPEDO {T{OR}PEDO*}
25 Raids trendy bars to seize cocaine essentially (7) INROADS {IN}{RO{cocAine}DS}
26 Standing in lakeside greenhouse (6) DEGREE [T]
27 Old males somehow raced in a zigzag course (8) SLALOMED*

DOWN
1   Sprinter primarily runs in discomfort due to this? (6) SPRAIN {Sp...r}{P{Runs}AIN} Semi&lit
2   Drink in depression, upset by losing a lawsuit (6) TIPPLE {PIT<=}{PLEa}
3   Poor overwhelmed by a cut in payment (9) ALLOWANCE {A}{L{LOW}ANCE}
4   Remain calm and decide not to go bald? (4,4,4,2) KEEP ONES HAIR ON [DD]
6   Kids consuming ecstasy in raves (5) RAGES {RAG{E}S}
7   Detective returns in subtle disguise (8) P?L?I?T? (Addendum - PALLIATE {PAL{TAIL<=}E} - See comments)
8   Song composed to secure a date? (8) SERENADE {SEREN{A}{D}E}
9   Hottest maiden, one working in restaurant for useful experience (5,2,3,4) GRIST TO THE MILL {HOTTEST+M+1}* in {GRILL}
15 Evil one starting to become real crazily intolerant (9) ILLIBERAL {ILL}{1}{Be...e}{REAL*}
16 Drinking booze, essentially hammered after one gets divorced (8) ISOLATED {S{boOze}LATED}<=>{1}
17 Staff catches one creep in high street (4,4) MAIN DRAG {MA{1}N}{DRAG}
19 Repelled attack to abduct son! Many put out of action (6) DISARM {RA{S}ID}<={M}
20 Heartless scoundrel grabbing miss locked up (6) CLOSED {Ca{LOSE}D}
22 Most elegantly portrayed stone monument (5) STELE [T]

Reference List

Girl = G, New = N, Old Boy = OB, Republican = R, Soldiers = OR, Runs = R, Ecstasy = E, Date = D, Maiden = M, Son = S, Many = M