Boss in educational activity mostly (4)
I believe the answer is:
stud
'boss' is the definition.
(both can mean a metal decoration)
'educational activity mostly' is the wordplay.
'educational' becomes 'stu' (I can't explain this - if you can you should give a lot more credence to this answer).
'activity' becomes 'do' (both can mean an event).
'mostly' means to remove the last letter (most but not all of the word).
'do' with its last letter removed is 'd'.
'stu'+'d'='STUD'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for stud that I've seen before include "Press fastener" , "Place of breeding" , "Part of a football boot" , "Boss; item of jewellery" , "old wooden post" .)