Head off crazy host (4)
I believe the answer is:
army
'host' is the definition.
(thesaurus)
'head off crazy' is the wordplay.
'head off' means to remove the first letter.
'crazy' becomes 'barmy' (I've seen this before).
'barmy' with its initial letter taken off is 'ARMY'.
(Other definitions for army that I've seen before include "Military land force" , "Large number of people" , "Ground forces" , "Large number (of ants?)" , "See 21" .)