Optimistic at university before defeat (6)
I believe the answer is:
upbeat
'optimistic' is the definition.
('upbeat' can be similar in meaning to 'optimistic')
'at university before defeat' is the wordplay.
'at university' becomes 'up'.
'before' says to put letters next to each other.
'defeat' becomes 'beat' (beating is a kind of defeating).
'up'+'beat'='UPBEAT'
(Other definitions for upbeat that I've seen before include "signal from sergeant, say, at end of bar?" , "Heartening" , "Conductor's beginning" , "Cheerful - positive about the future" , "Optimistic, positive" .)