Protestant archbishop's first proscription overturned — past it sadly (10)
I believe the answer is:
anabaptist
'protestant' is the definition.
(Anabaptist is an example)
'archbishop's first proscription overturned past it sadly' is the wordplay.
'archbishop's first' becomes 'a' (bst letter of 'archbishop').
'proscription' becomes 'ban' (synonyms).
'overturned' says the letters should be written backwards.
'sadly' indicates an anagram.
'ban' reversed gives 'nab'.
'past'+'it'='pastit'
'pastit' anagrammed gives 'aptist'.
'a'+'nab'+'aptist'='ANABAPTIST'
(Another definition for anabaptist that I've seen is " Protestant sect member".)