At school, nobody would miss him (3)
I believe the answer is:
sir
'would miss him' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'at school nobody' is the wordplay.
'at' is a charade indicator (letters next to each other).
'school' becomes 'S'.
'nobody' becomes 'ir' (I can't justify this - if you can you should believe this answer much more).
's'+'ir' is 'SIR'.
Can you help me to learn more?
(Other definitions for sir that I've seen before include "Male form of address" , "Baronet's title" , "Polite term of address for a man" , "Title given to 10 Across" , "address to teacher" .)