Positive about me before first performance (8)
I believe the answer is:
premiere
'first performance' is the definition.
(I've seen this before)
'positive about me before' is the wordplay.
'positive' becomes 'p'.
'about' becomes 're' (common abbreviation for 'with reference to').
'me' becomes 'mi' (both are spellings of the musical note).
'before' becomes 'ere' (usually literary).
'p'+'re'+'mi'+'ere'='PREMIERE'
(Other definitions for premiere that I've seen before include "First public performance of a play or movie" , "First ever public performance" , "Play's first public performance" , "First showing of play or film" , "First screening" .)