Speedy coppers participating in a raid (5)
I believe the answer is:
rapid
'speedy' is the definition.
(I know that speedy can be written as rapid)
'coppers participating in a raid' is the wordplay.
'coppers' becomes 'p'.
'participating in' means one lot of letters goes inside another.
'p' inserted inside 'raid' is 'RAPID'.
(Other definitions for rapid that I've seen before include "Prompt" , "Speedy (5)" , "Express" , "Zippy" , "Precipitate" .)