Shah's domain, once
I believe the answer is:
'shah's domain once' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for iran that I've seen before include "It used to be Persia" , "Country lying to south of Caspian sea" , "Nation" , "Tehran's country" , "Country where ayatollahs replaced a shah" .)