Set of beliefs, note, held by Conservative party (5)
I believe the answer is:
credo
'set of beliefs' is the definition.
(I've seen this before)
'note held by conservative party' is the wordplay.
'note' becomes 're' (**).
'held by' indicates putting letters inside.
'conservative' becomes 'c' (abbreviation commonly used in UK politics).
'party' becomes 'do' (do is a kind of party).
'c'+'do'='cdo'
're' going into 'cdo' is 'CREDO'.
(Other definitions for credo that I've seen before include "Statement of guiding aims" , "One's system of beliefs" , "Faith - I believe" , "Part of the Latin Mass" , "Doctrine or belief" .)