At school, the kids wouldn't miss him (3)
I believe the answer is:
sir
'wouldn't 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 the kids' is the wordplay.
'at' means one lot of letters go next to another.
'school' becomes 'S'.
'the kids' becomes 'ir' (I am not sure about this - if you are sure you should believe this answer much more).
's' next to 'ir' is 'SIR'.
Can you help me to learn more?
(Other definitions for sir that I've seen before include "Gentleman's form of address" , "Male title of respect" , "Formal and deferential address to a man" , "... ___ John Major" , "Feted fellow" .)