Part of circle a Roman Catholic joined (3)
I believe the answer is:
arc
'part of circle' is the definition.
('arc' is part of 'circle')
'a roman catholic joined' is the wordplay.
'roman catholic' becomes 'rc' ('r.c.' can be a synonym of 'roman catholic').
'joined' is a charade indicator (letters next to each other).
'a'+'rc'='ARC'
(Other definitions for arc that I've seen before include "Half-moon" , "Light" , "Part of a curve" , "Crescent" , "A curve; a discharge" .)