Drug user (6)
I believe the answer is:
addict
'drug user' is the definition.
(addict is a kind of drug user)
This is the entire clue.
(Other definitions for addict that I've seen before include "Junkie, dependant" , "One who is dependent on something, alcohol say" , "Fanatical devotee" , "Person with compulsive dependence on drugs" , "Bricklayer's tool" .)