2189: Offering

One unclued light is the name of a 15A activity requiring the 4A of three things. Answers to clues in italics must be treated before entry in the grid in such a way as to fulfil this requirement. Definitions of the resulting entries are supplied by the remaining unclued lights, one of which is hyphened.
 
Across
 
1    Tsar shaken by court disturbance

9    Taboo passed on, protecting tree (10)
11    Ape getting endless fruit (5)
12    Flat structure in back muscles (7)
14    Move briskly and strike (5)
16    Number in fine hospital improve (6)
24    Make application to keep left turn (4)
25    Pack round politician backing army (4)
27    Set last word in stone (7)
28    Result of trickling water in vessel after misfortune filling room (8)
33    Sham settlement with rat bodes ill

34    Rifles stand around (5)
35    Banter in mass meeting (5)
37    Stupid one in spot disrupting two clubs (7)
38    Broadcast now so faint (5)
39    Decorative style, new in gate, lacking power (10)
40    Break tile under ground (9)
41    Study hard core of punks in network (5)
 
Down
 
2    Monarch’s power projected in speech (6)
3    Be absorbed in wrong relationship (6, two words)
4    Poet, fool fears, lacking bias (6)
5    Essence in legal matter (4)
6    Sister upset about boorish songs (7)
7    Offender almost considered working (5)
8    Indian god of war in defeat (8)
10    Twist, involving criminal in dead end, entertained (13, three words)
13    Edgy bishop, abstainer consumed by anger (7)
15    Property later ruined and unknown (6)
17    United on trail that’s ending up in abode of giants (6)
18    Fellow with spirit in military cap (6)
19    Letter covering degree subjects (7)
20    Reds releasing English agent (6)
23    Shark formerly adept, after money (8)
26...