Old chap, one from sultanate (5)
I believe the answer is:
omani
'from sultanate' is the definition.
(from or relating to Oman)
'old chap one' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'chap' becomes 'man' (chap can mean a man).
'one' becomes 'i' (Roman numeral).
'o'+'man'+'i'='OMANI'
(Other definitions for omani that I've seen before include "Someone from Muscat" , "Speaker of Arabic" , "Native of one Arabian sultanate" , "one from the Middle East" , "Asian" .)