Boy's piece of music, unfinished (3)
I believe the answer is:
son
'boy's' is the definition.
('son' can be a synonym of 'boy')
'piece of music unfinished' is the wordplay.
'piece of music' becomes 'song' (song is a kind of piece of music).
'unfinished' means to remove the last letter.
'song' with its final letter removed is 'SON'.
(Other definitions for son that I've seen before include "Male descendant" , "Heir" , "Male scion" , "A relative" , "Native" .)