"Rigoletto" solo (4)
I believe the answer is:
'rigoletto solo' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for aria that I've seen before include "Elaborate accompanied song for solo voice" , "Air in an opera" , "Song, one from 'West Side Story' needing no introduction" , "Nessun Dorma, for example" , "Refrain" .)