Source of goodness in terrible song (5)
I believe the answer is:
dirge
'song' is the definition.
(dirge is a kind of song)
'source of goodness in terrible' is the wordplay.
'source of' says to take the initial letters.
'in' means one lot of letters goes inside another.
'terrible' becomes 'dire' ('dire' can be a synonym of 'terrible').
The first letter of 'goodness' is 'g'.
'g' put into 'dire' is 'DIRGE'.
(Other definitions for dirge that I've seen before include "part of funeral, perhaps" , "Funereal hymn" , "air?" , "dreary tune" , "A slow mournful song, lament for the dead" .)