Unaccompanied piece (4)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
solo
(Other definitions for solo that I've seen before include "Unaccompanied, musically or on adventure" , "Unaccompanied passage" , "On one's own, unaccompanied" , "Unaccompanied - card game" , "Alone, unaccompanied" .)