Talmudic adherent's backsliding sin couched in common parlance (9)
I believe the answer is:
rabbinist
'talmudic adherent's' is the definition.
(the Talmud is the body of Jewish law)
'backsliding sin couched in common parlance' is the wordplay.
'backsliding' says the letters should be written in reverse.
'couched in' indicates putting letters inside.
'common parlance' becomes 'rabbit' (rabbit can mean an informal conversation).
'sin' backwards is 'nis'.
'nis' inserted within 'rabbit' is 'RABBINIST'.
(Another definition for rabbinist that I've seen is " Being bound by Talmud".)