Outlaw head of drug ring (4)
I believe the answer is:
band
'ring' is the definition.
('band' can be a synonym of 'ring')
'outlaw head of drug' is the wordplay.
'outlaw' becomes 'ban' (banning is a kind of outlawing).
'head of' indicates taking the first letters.
The first letter of 'drug' is 'd'.
'ban'+'d'='BAND'
(Other definitions for band that I've seen before include "Pop group, perhaps" , "Orchestra" , "troop" , "A collection of musicians in ring" , "Musicians - gang" .)