Fanatical bishop in attack (5)
I believe the answer is:
rabid
'fanatical' is the definition.
(I know that fanatical can be written as rabid)
'bishop in attack' is the wordplay.
'bishop' becomes 'B' (abbreviation for bishop in chess).
'in' is an insertion indicator.
'attack' becomes 'raid' (raid is a kind of attack).
'b' inserted into 'raid' is 'RABID'.
(Other definitions for rabid that I've seen before include "extremist" , "Mad; raging" , "Ferocious" , "Furious like mad dog?" , "Frenzied" .)