Disorder causing delusions of persecution (8)
I believe the answer is:
paranoia
'disorder causing delusions of persecution' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for paranoia that I've seen before include "Unjustified suspicion and mistrust of others" , "Psychological disorder" , "Irrational fear or suspicion" , "Mental disorder characterised by delusions of persecution" , "Form of schizophrenia" .)