Leader in the mirror (4)
I believe the answer is:
emir
'leader' is the definition.
(I've seen this before)
'in the mirror' is the wordplay.
'in' says the answer is hidden in 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" , "leader in the East" , "Middle-Eastern ruler" , "Muslim ruler stuck in mire" , "Arab prince" .)