Sheer nonsense with unknown dissent (6)
I believe the answer is:
heresy
'dissent' is the definition.
(thesaurus)
'sheer nonsense with unknown' is the wordplay.
'nonsense with' is an anagram indicator.
'unknown' becomes 'y' (algebra).
'sheer' with its letters anagrammed with 'y' can make 'HERESY'.
(Other definitions for heresy that I've seen before include "Non-orthodox opinion" , "Opinion opposed to orthodox religious teachings" , "unorthodox beliefs" , "Divergence from orthodoxy" , "refusal to conform" .)