Matrimony possible after college (5)
I believe the answer is:
union
'matrimony' is the definition.
(I know that matrimony can be written as union)
'possible after college' is the wordplay.
'possible' becomes 'on' ('not on' can mean unacceptable or not possible).
'after' says to put letters next to each other.
'college' becomes 'uni' (short for university).
'on' after 'uni' is 'UNION'.
(Other definitions for union that I've seen before include "Wedding, say" , "Amalgamation" , "Association with a common purpose" , "match" , "Merger like marriage" .)