Country bordering Oman (5)
I believe the answer is:
yemen
'country bordering oman' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for yemen that I've seen before include "Aden is found here" , "Enemy (anag) - country of Arabia" , "Middle Eastern country" , "Arabian country, capital San'a" , "Country between the Red Sea and the Gulf of Aden" .)