Crazy British fighting men (5)
I believe the answer is:
barmy
'crazy' is the definition.
(I've seen this before)
'british fighting men' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'fighting men' becomes 'army' (armies fight).
'b'+'army'='BARMY'
(Other definitions for barmy that I've seen before include "Mad (like footballing "army")" , "round the bend" , "Mad (slang)" , "like Basil, arguably?" , "Extremely foolish" .)