Optimistic, cheerful (6)
I believe the answer is:
upbeat
This clue is a double definition.
'optimistic' is the first definition.
(similar in meaning)
'cheerful' is the second definition.
(I know that cheerful can be written as upbeat)
(Other definitions for upbeat that I've seen before include "in every bar" , "Cheerful - positive about the future" , "Optimistic - anacrusis" , "Optimistic (colloq.)" , "maestro communicates this to orchestra" .)