Part of circle a Catholic joined (3)
I believe the answer is:
arc
'part of circle' is the definition.
('arc' is part of 'circle')
'a catholic joined' is the wordplay.
'catholic' becomes 'rc' (I've seen this in other clues).
'joined' is a charade indicator (letters next to each other).
'a'+'rc'='ARC'
(Other definitions for arc that I've seen before include "Don't go straight" , "be rapidly discharged" , "Part of a circle's circumference" , "Electrical spark" , "Light" .)