Firm embracing socialist principles (5)
I believe the answer is:
credo
'principles' is the definition.
(I've seen this before)
'firm embracing socialist' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'embracing' means one lot of letters goes inside another.
'socialist' becomes 'red' (socialism is often associated with the colour red).
'co' going around 'red' is 'CREDO'.
(Other definitions for credo that I've seen before include "Mass movement" , "Apostles' beliefs" , "Statement of guiding aims" , "conviction" , "ideology" .)