It's his job to deal with troublesome pupils (7)
I believe the answer is:
oculist
'it's his job to deal with troublesome pupils' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
This is all the clue.
Can you help me to learn more?
(Other definitions for oculist that I've seen before include "One specialising in eye diseases" , "Optician" , "Specialist in defects and diseases of the eye" , "Person of vision" , "Treater of eyes" .)