Firm retaining radical set of beliefs (5)
I believe the answer is:
credo
'set of beliefs' is the definition.
(I've seen this before)
'firm retaining radical' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'retaining' is an insertion indicator.
'radical' becomes 'red' (I've seen this in other clues).
'co' going around 'red' is 'CREDO'.
(Other definitions for credo that I've seen before include "Words of believer" , "System of principles" , "Words to live by; belief" , "ideology" , "Part of the service" .)