Long story set about drug (5)
I believe the answer is:
yearn
'long' is the definition.
('yearn' can be a synonym of 'long')
'story set about drug' is the wordplay.
'story' becomes 'yarn' ('yarn' can be a synonym of 'story').
'set about' indicates putting letters inside.
'drug' becomes 'e' (e is another name for ecstasy).
'yarn' enclosing 'e' is 'YEARN'.
(Other definitions for yearn that I've seen before include "Have strong desire" , "Have a great longing" , "Long - pine" , "Mark of injury" , "lust" .)