Fanatical bishop in surprise attack (5)
I believe the answer is:
rabid
'fanatical' is the definition.
('rabid' can be a synonym of 'fanatical')
'bishop in surprise attack' is the wordplay.
'bishop' becomes 'b' (abbreviation used in chess).
'in' is an insertion indicator.
'surprise attack' becomes 'raid' (I've seen this before).
'b' going within 'raid' is 'RABID'.
(Other definitions for rabid that I've seen before include "Violent; hydrophobic" , "Ill from a mad dog bite?" , "Zealous" , "Raging (having been bitten by a dog?)" , "Extremely fanatical" .)