Short bloke has drug habit (4)
I believe the answer is:
robe
'habit' is the definition.
(I've seen this before)
'short bloke has drug' is the wordplay.
'short bloke' becomes 'Rob' (short for Robert; 'short' suggests a short form).
'has' means one lot of letters go next to another.
'drug' becomes 'e' (E is another name for ecstasy).
'rob'+'e'='ROBE'
(Other definitions for robe that I've seen before include "Clobber" , "Christening garment" , "Loose or formal garment" , "Loose garment for Boer" , "Gown or dress of office" .)