She is not herself in part (7)
I believe the answer is:
actress
'she' is the definition.
Both the definition and answer are singular nouns.
Perhaps there's an association between them I don't understand?
'not herself in part' is the wordplay.
I cannot really see how this works, but
'part' could be 'act' (I have seen 'Part of play ' mean 'act' so perhaps 'part' could also mean 'act') and 'act' is present in the answer.
This explanation may well be incorrect...
'is' acts as a link.
Can you help me to learn more?
(Other definitions for actress that I've seen before include "eg Emma Thompson" , "Theatrical female" , "Stage performer" , "Recasts (anag) - performer" , "one with a particular role?" .)