Stunner swallowing drug is a problem (6)
I believe the answer is:
teaser
'problem' is the definition.
(I know that teaser is a type of problem)
'stunner swallowing drug' is the wordplay.
'stunner' becomes 'taser' (weapon which stuns or shocks).
'swallowing' means one lot of letters goes inside another.
'drug' becomes 'e' (E is another name for ecstasy).
'taser' placed around 'e' is 'TEASER'.
'is a' acts as a link.
(Other definitions for teaser that I've seen before include "One who annoys people or cards wool" , "Tricky question; intriguing ad" , "You must be kidding!" , "Difficult question" , "Easter (anag) - puzzle" .)