Topless, dotty host (4)
I believe the answer is:
army
'host' is the definition.
(I've seen this before)
'topless dotty' is the wordplay.
'topless' means to remove the first letter.
'dotty' becomes 'barmy' (synonyms).
'barmy' with its first letter taken away is 'ARMY'.
(Other definitions for army that I've seen before include "Land-fighting force" , "fighters" , "Large host" , "See 21" , "Multitude" .)