Cheerful person (8)
(Other definitions for optimist that I've seen before include "One always tends to take a favourable view of things" , "Someone with a bright and hopeful nature" , "glass half-full type?" , "One wearing rose-tinted spectacles" , "Glass-half-full person" .)
'cheerful person' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for ray of sunshine that I've seen before include "Someone to make you smile" , "Cheerful person" , "joy to be with" , "One who makes others feel happy; beam" , "One cheering up other people" .)