Fanatical incursion capturing British (5)
I believe the answer is:
rabid
'fanatical' is the definition.
(I know that fanatical can be written as rabid)
'incursion capturing british' is the wordplay.
'incursion' becomes 'raid' (raid is a kind of incursion).
'capturing' means one lot of letters goes inside another.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'raid' going around 'b' is 'RABID'.
(Other definitions for rabid that I've seen before include "Fanatical - hydrophobic" , "Zealous" , "Infected with hydrophobia" , "Frenzied" , "Ferocious" .)