Soprano's solo, sometimes (4)
I believe the answer is:
aria
'soprano's solo sometimes' 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 "Operatic solo, duet, etc" , "Vocal solo in a cantata etc" , "Elaborate accompanied song for solo voice" , "Song, one from 'West Side Story' needing no introduction" , "Puccini item" .)