A disorder characterised by delusions (8)
I believe the answer is:
paranoia
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">a disorder characterised by delusions</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)