Finishing early, read up on boss (4)
I believe the answer is:
stud
'boss' is the definition.
(both can mean a metal decoration)
'finishing early read up on' is the wordplay.
'finishing early' means to remove the last letter.
'read up on' becomes 'study' (I've seen this before).
'study' with its final letter removed is 'STUD'.
(Other definitions for stud that I've seen before include "Fastening for collar" , "sort of stable" , "Large headed nail" , "Football boot attachment" , "Large-headed nail" .)