Boss requiring endless scrutiny (4)
I believe the answer is:
stud
'boss' is the definition.
(both can mean a metal decoration)
'endless scrutiny' is the wordplay.
'endless' means to remove the last letter.
'scrutiny' becomes 'study' (study is a kind of scrutiny).
'study' with its last letter taken away is 'STUD'.
'requiring' is the link.
(Other definitions for stud that I've seen before include "Virile man" , "Collar button or horse farm" , "Wall timber" , "Male pin-up" , "Horse-breeding farm" .)