Fanatic beheaded king (3)
I believe the answer is:
nut
'fanatic' is the definition.
(I've seen this before)
'beheaded king' is the wordplay.
'beheaded' means to remove the first letter (the head of the word is removed).
'king' becomes 'cnut' (I have seen 'Ancient king' mean 'cnut' so perhaps 'king' could also mean 'cnut').
'cnut' with its initial letter taken away is 'NUT'.
(Other definitions for nut that I've seen before include "Head (sl.)" , "Brazil, for instance" , "Fanatical enthusiast" , "Freak" , "Headcase" .)