Cheerful at college buffet (6)
I believe the answer is:
upbeat
'cheerful' is the definition.
('upbeat' can be a synonym of 'cheerful')
'at college buffet' is the wordplay.
'at college' becomes 'up'.
'buffet' becomes 'beat' (I've seen this before).
'up'+'beat'='UPBEAT'
(Other definitions for upbeat that I've seen before include "Relentlessly optimistic" , "maestro communicates this to orchestra" , "Optimistic - anacrusis" , "Of mood, cheerful and optimistic" , "Heartening" .)