He can see himself in the mirror (4)
I believe the answer is:
emir
'he can 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 a hidden word.
'EMIR' is hidden within 'the mirror'.
Can you help me to learn more?
(Other definitions for emir that I've seen before include "Ruler or chieftain in Arabia" , "Eastern chieftain" , "Arab leader" , "Arabian ruler" , "eader abroad?" .)