Frenzied British in attack (5)
I believe the answer is:
rabid
'frenzied' is the definition.
(I've seen this before)
'british in attack' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'in' indicates putting letters inside.
'attack' becomes 'raid' (raid is a kind of attack).
'b' going within 'raid' is 'RABID'.
(Other definitions for rabid that I've seen before include "Affected by 24 across" , "Ill from a mad dog bite?" , "Fanatical, infected" , "Furious like mad dog?" , "Ferocious" .)