Standard breached by Catholic faction (4)
I believe the answer is:
sect
'faction' is the definition.
('sect' can be a synonym of 'faction')
'standard breached by catholic' is the wordplay.
'standard' becomes 'set'.
'breached by' indicates putting letters inside.
'catholic' becomes 'c' (abbreviation as in RC for Roman Catholic).
'set' enclosing 'c' is 'SECT'.
(Other definitions for sect that I've seen before include "Heretical group" , "Dissenting body" , "Breakaway group" , "Small group with own religious beliefs" , "Religious faction" .)