Song sung alone (4)
I believe the answer is:
solo
'song sung alone' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for solo that I've seen before include "Unaccompanied flight" , "Performance by one person" , "Game for four" , "Alone - card game" , "Unaccompanied - card game" .)