Old servant one Muscat native? (5)
I believe the answer is:
omani
'muscat native?' is the definition.
(someone from Muscat in Oman)
'old servant one' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'servant' becomes 'man' (man is a kind of servant).
'one' becomes 'i' (roman numeral).
'o'+'man'+'i'='OMANI'
(Other definitions for omani that I've seen before include "Gulf Arab" , "One from Muscat" , "Naomi (anag) -- from Muscat?" , "Eg, a native of Muscat" , "Middle East national" .)