Socialist is entertained by firm statement of beliefs (5)
I believe the answer is:
credo
'statement of beliefs' is the definition.
(I've seen this before)
'socialist is entertained by firm' is the wordplay.
'socialist' becomes 'red' (red can informally mean a socialist).
'is entertained by' is an insertion indicator (entertain can mean to host or welcome in).
'firm' becomes 'co' (abbreviation for company).
'red' put into 'co' is 'CREDO'.
(Other definitions for credo that I've seen before include "Decor (anag.)" , "Dogma" , "set of ideas" , "One's system of beliefs" , "Part of the Latin Mass" .)