Chief stud (4)
I believe the answer is:
boss
I believe this is a double definition.
'chief' is the first definition.
(synonyms)
'stud' is the second definition.
(synonyms)
(Other definitions for boss that I've seen before include "Behave overbearingly" , "Gaffer, manager" , "Master - stud" , "Stud; employer" , "Manager (colloq.); stud" .)