Saying it isn't so (6)
I believe the answer is:
denial
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">saying it isn't so</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 denial that I've seen before include "Act of asserting that something alleged is not true" , "Repudiation" , "Refusal to accept an unpalatable truth" , "Refusal, abnegation" , "Renunciation" .)