Part of circle a churchgoer joined (3)
I believe the answer is:
arc
'part of circle' is the definition.
('arc' is part of 'circle')
'a churchgoer joined' is the wordplay.
'churchgoer' becomes 'rc' (abbreviation for 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 "Semi-circle" , "Bend" , "Portion of a circle" , "Curved section" , "Light" .)