British fighting force is nuts (5)
I believe the answer is:
barmy
'is nuts' is the definition.
('barmy' can be a synonym of 'nuts')
'british fighting force' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'fighting force' becomes 'army' (I've seen this before**).
'b'+'army'='BARMY'
(Other definitions for barmy that I've seen before include "Mad (like footballing "army")" , "Extremely foolish" , "like Basil, arguably?" , "Mad; crazy" , "-- Army, cricket supporters" .)