Good to interrupt dreadful sad song (5)
I believe the answer is:
dirge
'sad song' is the definition.
(I've seen this in another clue)
'good to interrupt dreadful' is the wordplay.
'good' becomes 'g' (abbreviation).
'to interrupt' indicates putting letters inside.
'dreadful' becomes 'dire' (synonyms).
'g' put within 'dire' is 'DIRGE'.
(Other definitions for dirge that I've seen before include "Funereal song" , "Keen; lament" , "Dreary song" , "feature of funeral" , "Sombre music" .)