A song coming up -- operatic piece (4)
I believe the answer is:
aria
'operatic piece' is the definition.
(solo in an opera)
'a song coming up' is the wordplay.
'song' becomes 'air' (air can mean a piece of music).
'coming up' says the letters should be written backwards (in a down clue, letters go up).
'air' in reverse letter order is 'ria'.
'a'+'ria'='ARIA'
(Other definitions for aria that I've seen before include "Refrain" , "Song such as "Your Tiny Hand is Frozen"" , "complex song" , "part of opera, maybe" , "Type of song" .)