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 a hidden word.
'EMIR' is hidden in the letters of '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" .)