Cheerful, optimistic (6)
I believe the answer is:
upbeat
I believe this clue is a double definition.
'cheerful' is the first definition.
(I know that cheerful can be written as upbeat)
'optimistic' is the second definition.
(similar in meaning)
(Other definitions for upbeat that I've seen before include "maestro communicates this to orchestra" , "Optimistic (colloq.)" , "Promising" , "Cheerful and encouraging, like lively music" , "Relentlessly optimistic" .)