Chieftain (4)
'chieftain' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for emir that I've seen before include "Title of various Muslim rulers" , "Islamic ruler" , "Arabian ruler" , "Local chief" , "Arab ruler" .)
'chieftain' is the definition.
(thesaurus)
This is the entire clue.
(Other definitions for ruler that I've seen before include "Drawing stick or monarch" , "One governing" , "Measuring strip" , "Queen, perhaps" , "that should go straight" .)