Firecracker’s quiet and slow when cap’s taken off (6)
I believe the answer is:
petard
'firecracker's' is the definition.
(I've seen this in another clue)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for petard that I've seen before include "Possession that one may be hoist with" , "Explosive device (with which one may be hoist)" , "charged case" , "A small bomb - don't be hoist by your own" , "Explosive device, one could have been hoist by their own, of old" .)