Fanatical attack stopped by British (5)
I believe the answer is:
rabid
'fanatical' is the definition.
(I know that fanatical can be written as rabid)
'attack stopped by british' is the wordplay.
'attack' becomes 'raid' (raiding is a kind of attacking).
'stopped by' indicates putting letters inside (stopping can mean blocking a gap).
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'raid' enclosing 'b' is 'RABID'.
(Other definitions for rabid that I've seen before include "Zealous" , "Frenzied" , "Crazy" , "Violent; hydrophobic" , "extremist" .)