Chieftain in the mirror (4)
I believe the answer is:
emir
'chieftain' is the definition.
(I've seen this before)
'in the mirror' is the wordplay.
'in' indicates the answer is hidden within the clue.
'EMIR' can be found hidden inside 'the mirror'.
(Other definitions for emir that I've seen before include "Leading role" , "Muslim chief" , "Arab chieftain" , "Islamic chieftain" , "Arabian ruler" .)