Sport doctor's box is missing old powder case (6)
I believe the answer is:
petard
'case' is the definition.
(I have seen 'Case of explosive' mean 'petard' so perhaps 'case' could also mean 'petard')
'sport doctor's box is missing old powder' is the wordplay.
I cannot really see how this works, but
'sport' could be 'pe' (physical education) and 'pe' is present in the answer.
'doctor' could be 'd' (eg in PhD, Doctor of Philosophy) and 'd' is located in the answer.
The remaining letters 'tar' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for petard that I've seen before include "A small bomb - don't be hoist by your own" , "Case of explosive" , "Something likely to explode" , "Explosive device prone to backfiring - figuratively (when hoisted?)" , "Hoist with one's own -" .)