Expression to introduce unknown belief not widely accepted (6)
I believe the answer is:
heresy
'expression' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for heresy that I've seen before include "Unorthodox view" , "Yes her (anag.)" , "Opinion opposed to orthodox religious teachings" , "Blasphemy" , "Unorthodox religious belief" .)