He could hardly fail to see himself in the mirror (4)
I believe the answer is:
emir
'he could hardly fail to 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 in the letters of 'the mirror'.
Can you help me to learn more?
(Other definitions for emir that I've seen before include "Title of various Muslim rulers" , "Arabian ruler" , "Leading role" , "Ruler in the Middle East and mire" , "Muslim ruler stuck in mire" .)