Communist bound by firm set of beliefs (5)
I believe the answer is:
credo
'set of beliefs' is the definition.
(I've seen this before)
'communist bound by firm' is the wordplay.
'communist' becomes 'red' (red is a kind of communist).
'bound by' means one lot of letters goes inside another (I've seen this in other clues).
'firm' becomes 'co' (a firm is a company).
'red' going into 'co' is 'CREDO'.
(Other definitions for credo that I've seen before include "One's system of beliefs" , "Guiding principles" , "mission statement" , "Statement of faith" , "System of principles" .)