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' says the answer is hidden in the clue.
'EMIR' is hidden within '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" .)