Opera tune (4)
I believe the answer is:
aria
'opera tune' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for aria that I've seen before include "Repeated strain" , "selection of opera" , "Vocal solo in a cantata etc" , "Song, one from 'West Side Story' needing no introduction" , "Elaborate accompanied song for solo voice" .)