Belief of socialist in company (5)
I believe the answer is:
credo
'belief' is the definition.
(credo is a kind of belief)
'socialist in company' is the wordplay.
'socialist' becomes 'red' (red is a kind of socialist).
'in' means one lot of letters goes inside another.
'company' becomes 'co' (abbreviation).
'red' inserted within 'co' is 'CREDO'.
'of' is the link.
(Other definitions for credo that I've seen before include "Apostles' beliefs" , "A statement of one's beliefs" , "Formal statement of belief" , "Guiding principles" , "Mass movement" .)