Cautious about Catholic slogan (3,3)
I believe the answer is:
war cry
'slogan' is the definition.
(war cry is a kind of slogan)
'cautious about catholic' is the wordplay.
'cautious' becomes 'wary' ('wary' can be similar in meaning to 'cautious').
'about' is an insertion indicator.
'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 "call for campaign" , "Rallying to fighters" , "Battle slogan" , "Word shouted in attach on enemy" , "Christian slogan" .)