Company adopts radical belief system (5)
I believe the answer is:
credo
'belief system' is the definition.
(I've seen this in another clue)
'company adopts radical' is the wordplay.
'company' becomes 'co' (abbreviation).
'adopts' means one lot of letters goes inside another.
'radical' becomes 'red' (red can mean a communist).
'co' enclosing 'red' is 'CREDO'.
(Other definitions for credo that I've seen before include "Faith - I believe" , "A statement of one's beliefs" , "Statement of faith" , "Guiding beliefs" , "Mass movement" .)