Pope's legacy: avoiding prison in retirement, simply (10)
I believe the answer is:
innocently
'simply' is the definition.
(thesaurus)
'pope's legacy avoiding prison in retirement' is the wordplay.
'pope' becomes 'innocent' (I've seen this before).
'avoiding' is a deletion indicator.
'prison' becomes 'cage' (cage is a kind of prison).
'in retirement' says the letters should be written in reverse (retire can mean to go back or backwards).
'cage' written backwards gives 'egac'.
'legacy' with 'egac' taken out is 'ly'.
'innocent'+'ly'='INNOCENTLY'
(Another definition for innocently that I've seen is " Artlessly".)