Gun with enamelled casing kept within college (5)
I believe the answer is:
gated
'kept within college' is the definition.
(gate can mean to confine a student to a college)
'gun with enamelled casing' is the wordplay.
'gun' becomes 'gat' (gat is a kind of gun).
'with' is a charade indicator (letters next to each other).
'casing' means to remove the middle letters (the outside letters are the case of the word).
'enamelled' with its middle removed is 'ed'.
'gat'+'ed'='GATED'
(Other definitions for gated that I've seen before include "Confined to school" , "Kept in as punishment" , "temporarily confined" , "Under curfew" , "Barred from leaving" .)