Cardinal, say, breaking firm set of beliefs (5)
I believe the answer is:
credo
'set of beliefs' is the definition.
(I've seen this before)
'cardinal say breaking firm' is the wordplay.
'cardinal say' becomes 'red' (cardinal is a kind of red).
'breaking' is an insertion indicator.
'firm' becomes 'co' (a firm is a company).
'red' going within 'co' is 'CREDO'.
(Other definitions for credo that I've seen before include "Guiding principles" , "Formal statement of belief" , "conviction" , "System of principles" , "One's system of beliefs" .)