Psychosis (8)
I believe the answer is:
paranoia
'psychosis' is the definition.
(I know that paranoia is a type of psychosis)
This is the entire clue.
(Other definitions for paranoia that I've seen before include "Feeling of persecution" , "State of mind involving delusions of persecution" , "Unjustified suspicion and mistrust of others" , "Irrational fear or suspicion" , "baseless fear" .)