Firm about communist beliefs (5)
I believe the answer is:
credo
'beliefs' is the definition.
(a credo is a statement of someone's beliefs)
'firm about communist' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'about' indicates putting letters inside.
'communist' becomes 'red' (red can informally mean communist).
'co' enclosing 'red' is 'CREDO'.
(Other definitions for credo that I've seen before include "piece of sacred music" , "Formal statement of beliefs" , "Mass movement" , "Doctrine or belief" , "One's system of beliefs" .)