Unorthodox belief (6)
I believe the answer is:
heresy
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">unorthodox belief</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)