Governor in the mirror (4)
I believe the answer is:
emir
'governor' is the definition.
(Muslim ruler)
'in the mirror' is the wordplay.
'in' indicates the answer is hidden within the clue.
'EMIR' is hidden within 'the mirror'.
(Other definitions for emir that I've seen before include "Islamic ruler" , "Arabian ruler" , "Middle Eastern ruler" , "Eastern title" , "Title of various rulers; I'm ER (anag.)" .)