Fanatic — monster (5)
I believe the answer is:
fiend
I believe this clue is a double definition.
'fanatic' is the first definition.
(I know that fanatic can be written as fiend)
'monster' is the second definition.
(I know that monster can be written as fiend)
(Other definitions for fiend that I've seen before include "Demon, ogre" , "addict" , "Evil supernatural being" , "Demon; cruel person" , "Evil spirit, demon" .)