Optimistic sign given to the orchestra (6)
I believe the answer is:
upbeat
'optimistic' is the definition.
(similar in meaning)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for upbeat that I've seen before include "Cheerful and encouraging, like lively music" , "Optimistic, positive" , "Heartening" , "Optimistic (colloq.)" , "Conductor's beginning" .)