Slip drug behind drinks (5)
I believe the answer is:
lapse
'slip' is the definition.
(lapsing is a kind of slipping)
'drug behind drinks' is the wordplay.
'drug' becomes 'e' (e is another name for ecstasy).
'behind' means one lot of letters go next to another.
'drinks' becomes 'laps' (lapping is a kind of drinking).
'e' put after 'laps' is 'LAPSE'.
(Other definitions for lapse that I've seen before include "Negligence" , "Error; expire" , "Decline into an inferior state" , "Fail in duty" , "Slip back" .)