Conservative referring to party’s belief set (5)
I believe the answer is:
credo
'belief set' is the definition.
'conservative referring to party's' is the wordplay.
'conservative' becomes 'c' (abbreviation commonly used in UK politics).
'referring to' becomes 're'.
'party' becomes 'do' (do is a kind of party).
'c'+'re'+'do'='CREDO'
(Other definitions for credo that I've seen before include "Part of the service" , "Faith - I believe" , "Statement of faith" , "ideology" , "music in church" .)