To sing about ultimate defeat can hurt (5)
I believe the answer is:
sting
'hurt' is the definition.
(if something stings, it hurts)
'to sing about ultimate defeat' is the wordplay.
'about' is an insertion indicator.
'ultimate defeat' becomes 't' (I can't explain this - if you can you should give a lot more credence to this answer).
'sing' going around 't' is 'STING'.
'can' is the link.
Can you help me to learn more?
(Other definitions for sting that I've seen before include "bit of deception" , "Entrapment operation; sharp pain" , "Con: leader singer of Police" , "Hurt; smart" , "con trick" .)