One time at college (4)
I believe the answer is:
unit
'one' is the definition.
('unit' can be a synonym of 'one')
'time at college' is the wordplay.
'time' becomes 't'.
'at' is a charade indicator (letters next to each other) (I've seen this in other clues).
'college' becomes 'uni' (I've seen this in other clues).
't' after 'uni' is 'UNIT'.
(Other definitions for unit that I've seen before include "Element or part" , "Self-contained section" , "squad" , "Single undivided entity" , "One part of a whole" .)