Opposed to Protestantism to some extent (4)
I believe the answer is:
anti
'opposed to' is the definition.
(I've seen this before)
'protestantism to some extent' is the wordplay.
'to some extent' indicates a hidden word.
'ANTI' is hidden in the letters of 'protestantism'.
(Other definitions for anti that I've seen before include "Not favouring" , "(Person who is) not in favour" , "one opposing?" , "Hostile" , "Definitely not pro" .)