Company accepting communist belief (5)
I believe the answer is:
credo
'belief' is the definition.
(I know that credo is a type of doctrine)
'company accepting communist' is the wordplay.
'company' becomes 'co' (abbreviation).
'accepting' is an insertion indicator (inserted letters are accepted).
'communist' becomes 'red' (red can informally mean communist).
'co' placed around 'red' is 'CREDO'.
(Other definitions for credo that I've seen before include "Faith or belief" , "Apostles' beliefs" , "System of principles" , "music in church" , "Part of the service" .)