Firm about socialist beliefs (5)
I believe the answer is:
credo
'beliefs' is the definition.
(I've seen this before)
'firm about socialist' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'about' means one lot of letters goes inside another.
'socialist' becomes 'red' (red is a kind of socialist).
'co' placed around 'red' is 'CREDO'.
(Other definitions for credo that I've seen before include "mission statement" , "Faith - I believe" , "A statement of one's beliefs" , "Statement of guiding aims" , "System of principles" .)