Host is very foolish wasting billions (4)
I believe the answer is:
army
'host' is the definition.
(I've seen this in another clue)
'very foolish wasting billions' is the wordplay.
'very foolish' becomes 'barmy' (barmy can mean mad).
'wasting' is a deletion indicator.
'billions' becomes 'b'.
'barmy' with 'b' taken out is 'ARMY'.
'is' acts as a link.
(Other definitions for army that I've seen before include "Large number (of ants?)" , "See 17" , "Large military force" , "Large host" , "fighters" .)