Arabian leader? (4)
'arabian leader' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for emir that I've seen before include "Top man" , "Arab ruler" , "Leading role" , "Islamic chieftain" , "Ruler in the Middle East and mire" .)
(Other definitions for rein that I've seen before include "one's attached to a bit" , "Animal controller" , "leading article?" , "A means of control" , "Part of a horses's harness" .)