Belief expressed by socialist in company (5)
I believe the answer is:
credo
'belief' is the definition.
(I know that credo is a type of doctrine)
'socialist in company' is the wordplay.
'socialist' becomes 'red' (red can informally mean a socialist).
'in' indicates putting letters inside.
'company' becomes 'co' (abbreviation).
'red' inserted inside 'co' is 'CREDO'.
'expressed by' acts as a link.
(Other definitions for credo that I've seen before include "Words of believer" , "Faith or belief" , "System of principles" , "Doctrine or belief" , "ideology" .)