Song for one singer (4)
I believe the answer is:
solo
'song for one singer' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for solo that I've seen before include "Piece for one performer" , "(Piece performed) on one's own" , "Aline in Oslo" , "By oneself" , "Whist variant" .)