His pupils wouldn't miss him (3)
I believe the answer is:
sir
'his pupils wouldn't miss' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'him' is the wordplay.
I cannot really see how this works, but
'him' could be 'si' and 'si' is found within the answer.
A single letter 'r' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for sir that I've seen before include "Title given to baronet" , "Respectful address for a man" , "Term of address for a gentleman" , "Poet and novelist" , "Male title of respect" .)