Polite pupils possibly please him (3)
I believe the answer is:
sir
'polite pupils possibly please him' 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 quite see how this works, but
'him' could be 'si' and 'si' is located in the answer.
A single letter 'r' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for sir that I've seen before include "... ___ John Major" , "Title of 1 across" , "Honourable title" , "Male form of address" , "Term of address for a gentleman" .)