A disorder characterised by delusions (8)
I believe the answer is:
paranoia
'a disorder characterised by delusions' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for paranoia that I've seen before include "State of mind involving delusions of persecution" , "Persecution mania" , "Delusion of being persecuted" , "Disorder characterized by delusions of persecution" , "baseless fear" .)