Despicable chap injecting drug (4)
I believe the answer is:
mean
'despicable' is the definition.
(I've seen this before)
'chap injecting drug' is the wordplay.
'chap' becomes 'man' (I've seen this before**).
'injecting' indicates putting letters inside.
'drug' becomes 'e' (e is another name for ecstasy).
'man' enclosing 'e' is 'MEAN'.
(Other definitions for mean that I've seen before include "design" , "Stingy; five Dn" , "spell" , "Intend; unkind" , "Common" .)