Cynical to start with concerning party dogma (5)
I believe the answer is:
credo
'dogma' is the definition.
(both can mean beliefs)
'cynical to start with concerning party' is the wordplay.
'to start with' suggests taking the first letters.
'concerning' becomes 're' (both can mean on a particular topic).
'party' becomes 'do' (do is a kind of party).
The first letter of 'cynical' is 'c'.
'c'+'re'+'do'='CREDO'
(Other definitions for credo that I've seen before include "Statement of faith" , "Statement of guiding aims" , "Formal statement of belief" , "Guiding beliefs" , "Part of the service" .)