Arab in later years reversed narcissism (8)
I believe the answer is:
egomania
'narcissism' is the definition.
(I've seen this in another clue)
'arab in later years reversed' is the wordplay.
'arab' becomes 'Omani' (someone from Oman).
'in' is an insertion indicator.
'later years' becomes 'age' (old age).
'reversed' is a reversal indicator.
'age' in reverse letter order is 'ega'.
'omani' inserted into 'ega' is 'EGOMANIA'.
(Other definitions for egomania that I've seen before include "Obsessive self-conceit" , "Thing about me" , "Extreme self-centredness" , "Obsessive self-regard" , "Obsessive self-centredness" .)