Republican on about mother being Catholic (5)
I believe the answer is:
roman
'being catholic' is the definition.
(I've seen this before)
'republican on about mother' is the wordplay.
'republican' becomes 'r' (abbreviation).
'about' means one lot of letters goes inside another.
'mother' becomes 'ma' (ma can mean mother).
'on' going around 'ma' is 'oman'.
'r'+'oman'='ROMAN'
(Other definitions for roman that I've seen before include "eg Julius Caesar" , "Style of upright type" , "Type of candle, nose etc" , "A novel" , "One like Julius Caesar" .)