Revolutionary firm about belief (5)
I believe the answer is:
credo
'belief' is the definition.
(credo is a kind of belief)
'revolutionary firm about' is the wordplay.
'revolutionary' becomes 'red'.
'firm' becomes 'co' (a firm is a company).
'about' is an insertion indicator.
'red' placed into 'co' is 'CREDO'.
(Other definitions for credo that I've seen before include "Part of the Latin Mass" , "Statement of guiding aims" , "Belief system" , "Dogma" , "One's system of beliefs" .)