Old chap in Arab country (4)
I believe the answer is:
oman
'arab country' is the definition.
(I've seen this before)
'old chap' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'chap' becomes 'man' (chap can mean a man).
'o'+'man'='OMAN'
'in' acts as a link.
(Other definitions for oman that I've seen before include "Arabian peninsula country" , "Sultanate on the south-east coast of the Arabian Peninsula" , "Former Gulf country, Muscat and --" , "Gulf sultanate" , "Nation" .)