Company holding left-wing set of beliefs (5)
I believe the answer is:
credo
'set of beliefs' is the definition.
(I've seen this before)
'company holding left-wing' is the wordplay.
'company' becomes 'co' (abbreviation).
'holding' means one lot of letters goes inside another.
'left wing' becomes 'red' (I've seen this in other clues).
'co' placed around 'red' is 'CREDO'.
(Other definitions for credo that I've seen before include "mission statement" , "Doctrine or belief" , "System of principles" , "set of ideas" , "Statement of faith" .)