A hat like this could belong to a Roman Catholic (3)
I believe the answer is:
red
This clue might be a double definition.
'a hat like this could belong' is the first definition.
The definition suggests a singular noun which matches the answer.
'roman catholic' is the second definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'to a' acts as a link.
Can you help me to learn more?
(Other definitions for red that I've seen before include "Colour of 1 across" , "Glowing" , "Colour for danger" , "Very angry, seeing this" , "Florid" .)