Socialist that's firm about statement of principles (5)
I believe the answer is:
credo
'statement of principles' is the definition.
(I've seen this before)
'socialist that's firm about' is the wordplay.
'socialist' becomes 'red' (red can informally mean a socialist).
'that's firm' becomes 'co' (a firm is a company).
'about' is an insertion indicator.
'red' placed within 'co' is 'CREDO'.
(Other definitions for credo that I've seen before include "Part of the service" , "Words to live by; belief" , "Decor (anag.)" , "A statement of one's beliefs" , "Section of mass" .)