Say Arab has a personality disorder (8)
I believe the answer is:
egomania
'personality disorder' is the definition.
Both the answer and definition are singular nouns.
Perhaps you can see an association between them that I don't see?
'say arab has a' is the wordplay.
'say' becomes 'eg' (both can mean 'for example').
'arab' becomes 'Omani' (someone from Oman).
'has' means one lot of letters go next to another.
'eg'+'omani'+'a'='EGOMANIA'
Can you help me to learn more?
(Other definitions for egomania that I've seen before include "Obsessive self-centredness" , "Excessive self-centredness" , "Obsessive self-interest" , "having no thought for others" , "Abnormally excessive opinion of oneself, quite mad" .)