Sets of opinions society rejected, socialist getting squashed (6)
I believe the answer is:
credos
'sets of opinions' is the definition.
(a credo is a statement of one's beliefs)
'society rejected socialist getting squashed' is the wordplay.
'society' becomes 'soc' (abbreviation in the names of organisations).
'rejected' is a reversal indicator.
'socialist' becomes 'red' (socialism is often associated with the colour red).
'getting squashed' is an insertion indicator (inserted letters get squashed inside).
'soc' backwards is 'cos'.
'cos' enclosing 'red' is 'CREDOS'.
(Other definitions for credos that I've seen before include "Musical movements" , "Scored with beliefs or principles" , "Scored with tenets" , "Sections of masses" .)