One who looks on the bright side (8)
I believe the answer is:
optimist
'one who looks on the bright side' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for optimist that I've seen before include "Glass-half-full person" , "glass half-full type?" , "I expect more?" , "Hopeful and confident person" , "One always believes the glass is half-full" .)