Boss of half the college's learners? (4)
I believe the answer is:
stud
'boss' is the definition.
(both can mean a metal decoration)
'half the college's learners?' is the wordplay.
'half the' indicates taking half.
'college's learners?' becomes 'students' (people studying at college).
'students' with half the letters taken is 'STUD'.
'of' acts as a link.
(Other definitions for stud that I've seen before include "Collar button or animal farm" , "Small piece of jewellery" , "Collar-fastener" , "Collar fastener - form of poker" , "Fastening for collar" .)