Mournful song's awful, including its ending? (5)
I believe the answer is:
dirge
'mournful song's' is the definition.
(piece of music lamenting the dead)
I don't understand how the rest of the clue works.
(Other definitions for dirge that I've seen before include "feature of funeral" , "part of funeral, perhaps" , "Mournful chant" , "Funereal music" , "Requiem" .)