A piece of light music? (5,4)
I believe the answer is:
torch song
'a piece' is the definition.
(torch song is a kind of piece)
'of light music?' is the wordplay.
'of light' becomes 'torch' (I can't explain this - if you can you should believe this answer much more).
'music?' becomes 'song' (song is a kind of music).
'torch'+'song'='TORCH-SONG'
Can you help me to learn more?
(Other definitions for torch song that I've seen before include "form of the blues?" , "effusion of love" , "Ballad of unrequited love" , "Sentimental number" , "Expression of unrequited love" .)