Not a real Pope - just one acting nobly at the outset (4)
I believe the answer is:
joan
'not a real pope' is the definition.
I can't tell whether this defines the answer.
'just one acting nobly at the outset' is the wordplay.
'at the outset' indicates taking the first letters (the outset is the beginning).
The first letters of 'just one acting nobly' is 'JOAN'.
Can you help me to learn more?
(Other definitions for joan that I've seen before include "Orleans girl" , ". . . . of Arc, French warrior saint" , "- Sutherland, Australian soprano" , "'Saint - - ' (Shaw play)" , "First name of the Maid of Orleans" .)