Operatic number (4)
I believe the answer is:
aria
'operatic number' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for aria that I've seen before include "Summertime perhaps" , "part of opera, maybe" , "Operatic vocal" , "Song for solo voice from an oratorio, cantata etc" , "Puccini item" .)