Delusions of persecution (8)
I believe the answer is:
paranoia
'delusions of persecution' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for paranoia that I've seen before include "Disorder characterised by delusions of persecution or grandeur" , "Mental disorder characterised by delusions of persecution" , "Irrational fear or suspicion" , "Disorder characterized by delusions of persecution" , "Feeling of persecution" .)