Say deliver backing for sorrowful song (5)
I believe the answer is:
dirge
'sorrowful song' is the definition.
(I've seen this in another clue)
'say deliver backing' is the wordplay.
'say' becomes 'eg' (both can mean 'for example').
'deliver' becomes 'rid' (e.g. deliver us from evil).
'backing' says the letters should be written backwards.
'eg'+'rid'='egrid'
'egrid' written backwards gives 'DIRGE'.
'for' is the link.
(Other definitions for dirge that I've seen before include "Keen; lament" , "Funeral song" , "Song or hymn of mourning" , "Funeral music" , "Slow, sad song" .)