Boss a form of poker (4)
I believe the answer is:
stud
I believe this clue is a double definition.
'boss' is the first definition.
(synonyms)
'a form of poker' is the second definition.
(I've seen this before)
(Other definitions for stud that I've seen before include "Fastening for collar" , "sort of stable" , "Horse-breeding establishment; jewellery item" , "Virile man" , "who believes he can keep it up" .)