Optimistic conductor may start with it (6)
I believe the answer is:
I believe this is a double definition.
'optimistic' is the first definition.
(similar in meaning)
'conductor may start with it' is the second definition.
(as in an upbeat in music)
(Other definitions for upbeat that I've seen before include "Cheerful and sanguine - as in music?" , "Promising" , "Cheerfully optimistic" , "bright and cheery" , "Optimistic (colloq.)" .)