Horse circling male groom (4)
I believe the answer is:
comb
'groom' is the definition.
(combing is a kind of grooming)
'horse circling male' is the wordplay.
'horse' becomes 'cob' (cob is a kind of horse).
'circling' indicates putting letters inside (circling can mean going around or containing).
'male' becomes 'm' (common abbreviation).
'cob' enclosing 'm' is 'COMB'.
(Other definitions for comb that I've seen before include "Crest on the head of some birds" , "Used to tidy hair" , "Search well; bird's crest" , "Toothed implement - search (the area)" , "Toothed instrument" .)