Belief contrary to the orthodox (6)
I believe the answer is:
heresy
'belief contrary to the orthodox' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for heresy that I've seen before include "unorthodox beliefs" , "Iconoclasm" , "Unorthodox religious belief" , "Opinion contrary to orthodox belief" , "Opinion opposed to orthodox religious teachings" .)