Regularly scorned party's set of beliefs (5)
I believe the answer is:
credo
'set of beliefs' is the definition.
(I've seen this before)
'regularly scorned party's' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'party' becomes 'do' (do is a kind of party).
The alternate letters of 'scorned' are 'cre'.
'cre'+'do'='CREDO'
(Other definitions for credo that I've seen before include "I believe in Latin" , "A statement of one's beliefs" , "System of principles" , "One's system of beliefs" , "Formal statement of belief" .)