Careful to adopt Catholic slogan
I believe the answer is:
war cry
'slogan' is the definition.
(war cry is a kind of slogan)
'careful to adopt catholic' is the wordplay.
'careful' becomes 'wary' ('wary' can be similar in meaning to 'careful').
'to adopt' indicates putting letters inside.
'catholic' becomes 'rc' (I've seen this in other clues).
'wary' placed around 'rc' is 'WAR-CRY'.
(Other definitions for war cry that I've seen before include "rallying call" , "Battle slogan" , "Christian slogan" , "slogan, for example" , "Religious publication" .)