Ruler in the mirror (4)
I believe the answer is:
emir
'ruler' is the definition.
(historical term for a Muslim ruler)
'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 "Title of various Muslim rulers" , "Eastern ruler in mire" , "Arab prince" , "Arab leader" , "Islamic chieftain" .)