He's not himself (12)
I believe the answer is:
impersonator
'he's not himself' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he's' is the wordplay.
I cannot really understand how this works, but
'he' could be 'son' and 'son' is found within the answer.
The remaining letters 'imperator' is a valid word 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 impersonator that I've seen before include "ape" , "Impressionist" , "One's often acting" , "One pretending to be another" , "Mimic" .)