One suffering endlessly evil spell (6)
I believe the answer is:
victim
'one suffering' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'endlessly evil spell' is the wordplay.
'endlessly' means to remove the last letter.
'evil' becomes 'vice' (vice is a kind of evil).
'spell' becomes 'tim' (I can't justify this - if you can you should give a lot more credence to this answer).
'vice' with its last letter taken away is 'vic'.
'vic'+'tim'='VICTIM'
Can you help me to learn more?
(Other definitions for victim that I've seen before include "Casualty of a crime" , "sacrificed animal" , "One suffers from adverse circumstances" , "Person injured or killed in accident" , "One killed, hurt or duped" .)