Short bloke with drug habit (4)
I believe the answer is:
robe
'habit' is the definition.
(I've seen this before)
'short bloke with drug' is the wordplay.
'short bloke' becomes 'rob' (short for Robert; 'short' suggests an abbreviation).
'with' says to put letters next to each other.
'drug' becomes 'e' (E is another name for ecstasy).
'rob'+'e'='ROBE'
(Other definitions for robe that I've seen before include "Ceremonial garb" , "Clobber" , "Loose garment for Boer" , "Dressing-gown" , "Formal vestment" .)