Ready to accept Catholic group (4)
I believe the answer is:
sect
'group' is the definition.
(I've seen this in other clues)
'ready to accept catholic' is the wordplay.
'ready to' becomes 'set' ('set' can be a synonym of 'ready to').
'accept' is an insertion indicator.
'catholic' becomes 'c' (abbreviation as in RC for Roman Catholic).
'set' enclosing 'c' is 'SECT'.
(Other definitions for sect that I've seen before include "Dissenting clique of a religion" , "Religious faction" , "Subdivision of a larger religious group" , "Dissenting division of a larger religious group" , "Subdivision of a religion" .)