He could see himself in the mirror (4)
I believe the answer is:
emir
'he could see himself' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'in the mirror' is the wordplay.
'in' indicates the answer is hidden within the clue.
'EMIR' is hidden within 'the mirror'.
Can you help me to learn more?
(Other definitions for emir that I've seen before include "Middle-Eastern ruler" , "Leading role" , "Eastern title" , "Islamic title" , "leader in the East" .)