Soldiers being very foolish (not British) (4)
I believe the answer is:
army
'soldiers' is the definition.
(group of soldiers)
'very foolish not british' is the wordplay.
'very foolish' becomes 'barmy' (barmy can mean mad).
'not' suggests deleting specific letters.
'british' becomes 'b' (abbreviation e.g. in BBC).
'barmy' with 'b' taken out is 'ARMY'.
'being' acts as a link.
(Other definitions for army that I've seen before include "service" , "Military unit" , "Land-fighting force" , "See 7" , "Large host" .)