What you own to be a need for exorcism (10)
I believe the answer is:
possession
'need for exorcism' is the definition.
The answer and definition can be both states of affairs as well as being singular nouns.
Maybe you can see a link between them that I can't see?
'what you own to be a' is the wordplay.
I cannot really see how this works, but
'own' could be 'possess' (synonyms) and 'possess' is found within the answer.
The remaining letters 'ion' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for possession that I've seen before include "Haunting" , "title" , "Ownership" .)