Fanatical devotee (6)
I believe the answer is:
addict
'fanatical devotee' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for addict that I've seen before include "One hooked on drugs" , "One hooked on something" , "Regular drug user" , "Slave to a chemical substance" , "One dependent on drugs" .)