Nation on the Gulf of Oman (4)
I believe the answer is:
iran
'nation on the gulf of oman' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for iran that I've seen before include "It used to be Persia" , "oil supplier" , "Modern name for Persia" , "Persia some time ago" , "Middle Eastern country" .)