College had reduced students (4)
I believe the answer is:
hall
'college' is the definition.
'had reduced students' is the wordplay.
'reduced' means to remove the last letter.
'students' becomes 'll' (an abbreviation for learner driver written twice).
'had' with its final letter taken off is 'ha'.
'ha'+'ll'='HALL'
(Other definitions for hall that I've seen before include "See 1" , "Large country house" , "College dining room" , "Large public building" , "Entrance area" .)