Host is extremely foolish going topless (4)
I believe the answer is:
army
'host' is the definition.
(both can mean a large number of people)
'extremely foolish going topless' is the wordplay.
'extremely foolish' becomes 'barmy' (barmy means silly or mad).
'going topless' means to remove the first letter.
'barmy' with its first letter taken away is 'ARMY'.
'is' acts as a link.
(Other definitions for army that I've seen before include "Fighting unit" , "Horde" , "See 6" , "Multitude" , "Large number of people" .)