Horse farm’s boss (4)
I believe the answer is:
stud
This clue is a triple definition!
'horse' is the first definition.
(thesaurus)
'farm's' is the second definition.
(stud farm is a kind of farm)
'boss' is the third definition.
(synonyms)
(Other definitions for stud that I've seen before include "Press to secure this" , "Playboy" , "Style of poker" , "Farm for breeding horses" , "Dot" .)