"Tosca" highlight (4)
I believe the answer is:
aria
'tosca highlight' 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 "Song from an opera (4)" , "Song, one from 'West Side Story' needing no introduction" , "Operatic air" , "Operatic vocal" , "Nessun Dorma, for example" .)