This will affect many with ill feeling (8)
I believe the answer is:
epidemic
'this will affect many with ill feeling' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
This is all the clue.
Can you help me to learn more?
(Other definitions for epidemic that I've seen before include "Prevailing" , "Prevalent disease" , "Plague" , "health emergency" , "Widespread occurrence of a disease" .)