Unorthodox belief (6)
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I believe the answer is:
heresy
'unorthodox belief' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for heresy that I've seen before include "Heterodoxy" , "Opinion contrary to orthodox belief" , "Opinion contrary to accepted religious beliefs" , "Deviation from orthodox religious thinking" , "Belief that's anathema to the orthodox" .)