Old person, one from Arab state (5)
I believe the answer is:
omani
'arab state' is the definition.
Although both the answer and definition are adjectives, I don't see how they can define each other.
'old person one' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'person' becomes 'man' (man is a kind of person).
'one' becomes 'i' (Roman numeral).
'o'+'man'+'i'='OMANI'
'from' acts as a link.
Can you help me to learn more?
(Other definitions for omani that I've seen before include "Someone from Muscat" , "one from the Middle East" , "Gulf Arab" , "Speaker of Arabic" , "M East person" .)