Part of circle of a Roman Catholic (3)
I believe the answer is:
arc
'part of circle' is the definition.
(part of the circumference of a circle)
'a roman catholic' is the wordplay.
'roman catholic' becomes 'RC' (abbreviation for Roman Catholic).
'a'+'rc'='ARC'
'of' is the link.
(Other definitions for arc that I've seen before include "Turn" , "circle, partly" , "Part of curved line" , "Something used to weld" , "Portion of a circle" .)