By oneself, alone (4)
I believe the answer is:
solo
I believe this clue is a double definition.
'by oneself' is the first definition.
(I've seen this in another clue)
'alone' is the second definition.
(as in a musical solo)
(Other definitions for solo that I've seen before include "Unaccompanied flight" , "Han - (Star Wars); fly unaccompanied" , "Performance by one person" , "On one's own, unaccompanied" , "Go it alone" .)