Gainsay (10)
I believe the answer is:
contradict
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">gainsay</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 contradict that I've seen before include "Deny" , "Assert the opposite of (a statement)" , "Argue against" , "Refute" , "Disagree" .)