Optimistic, buoyant (6)
I believe the answer is:
upbeat
I believe this clue is a double definition.
'optimistic' is the first definition.
(similar in meaning)
'buoyant' is the second definition.
(thesaurus)
(Other definitions for upbeat that I've seen before include "Promising" , "Conductor's beginning" , "Cheerful and encouraging, like lively music" , "Of mood, cheerful and optimistic" , "Cheerfully optimistic" .)