Circle an island sultanate (4)
I believe the answer is:
oman
'sultanate' is the definition.
(I've seen this before)
'circle an island' is the wordplay.
'circle' becomes 'o'.
'an island' becomes 'man' (Man is an example).
'o'+'man'='OMAN'
(Other definitions for oman that I've seen before include "Arabian peninsula country" , "Arabian country, capital Muscat" , "Land" , "Arab state" , "Sultanate on the south-east coast of the Arabian Peninsula" .)