Host, daft going topless (4)
I believe the answer is:
'host' is the definition.
'daft going topless' is the wordplay.
'daft' becomes 'barmy' ('barmy' can be a synonym of 'daft').
'going topless' means to remove the first letter.
'barmy' with its first letter taken off is 'ARMY'.
(Other definitions for army that I've seen before include "Organisation for soldiering" , "A great number (of soldiers?)" , "See 17" , "Land-fighting force" , "Horde" .)