Old fellow’s country (4)
I believe the answer is:
oman
'country' is the definition.
(Oman is an example)
'old fellow's' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'fellow' becomes 'man' (fellow can mean a man).
'o'+'man'='OMAN'
(Other definitions for oman that I've seen before include "Arabian country, capital Muscat" , "Arabian peninsula country" , "Arab state" , "Gulf sultanate" , "Middle Eastern country" .)