Borders closed off in country of sultanate (5)
I believe the answer is:
omani
'borders closed off' is the definition.
I can't tell whether this definition defines the answer.
'country of sultanate' is the wordplay.
'country' becomes 'Oman' (Oman is an example).
'of sultanate' becomes 'i' (I can't justify this - if you can you should believe this answer much more).
'oman'+'i'='OMANI'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for omani that I've seen before include "One from Muscat" , "Inhabitant of Middle Eastern sultanate" , "Asian" , "Native of one Arabian sultanate" , "coming from the Middle East" .)