Ruler seen in reversible mirror (4)
I believe the answer is:
emir
'ruler' is the definition.
(historical term for a Muslim ruler)
'in reversible mirror' is the wordplay.
'in' indicates the answer is hidden within the clue.
'EMIR' can be found hidden inside 'reversible mirror'.
'seen' is the link.
(Other definitions for emir that I've seen before include "leader in the East" , "Title of various Muslim rulers" , "Arabian ruler" , "Muslim ruler stuck in mire" , "Middle-Eastern ruler" .)