One that sheds light on Catholic vice (5)
I believe the answer is:
clamp
'vice' is the definition.
(I've seen this before)
'one that sheds light on catholic' is the wordplay.
'one that sheds light' becomes 'lamp'.
'on' means one lot of letters go next to another.
'catholic' becomes 'c' (abbreviation as in RC for Roman Catholic).
'lamp' put after 'c' is 'CLAMP'.
(Other definitions for clamp that I've seen before include "Device for holding things tightly" , "Gripping tool" , "Fix firmly" , "Pile of bricks" , "pile of turf" .)