Fellow backing old sultanate (4)
I believe the answer is:
oman
'sultanate' is the definition.
(Sultanate of Oman, Middle Eastern country)
'fellow backing old' is the wordplay.
'fellow' becomes 'man' (fellow can mean a man).
'backing' means one lot of letters go next to another.
'old' becomes 'o' (abbreviation).
'man' put after 'o' is 'OMAN'.
(Other definitions for oman that I've seen before include "Middle Eastern country" , "monarchy" , "Muscat's country" , "Former Gulf country, Muscat and --" , "Sultanate, capital Muscat" .)