Communist firm about belief (5)
I believe the answer is:
credo
'belief' is the definition.
(credo is a kind of belief)
'communist firm about' is the wordplay.
'communist' becomes 'red' (red can informally mean communist).
'firm' becomes 'co' (abbreviation for company).
'about' means one lot of letters goes inside another.
'red' placed into 'co' is 'CREDO'.
(Other definitions for credo that I've seen before include "ideology" , "Belief system" , "Set of beliefs about leftist circle" , "Doctrine or belief" , "piece of sacred music" .)