Sills song (4)
I believe the answer is:
aria
'sills song' 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 "Operatic solo, duet, etc" , "complex song" , "Operatic solo performance" , "Song such as "Your Tiny Hand is Frozen"" , "Operatic part" .)