Counter; deny (6)
I believe the answer is:
refute
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">counter</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ >'<span class="explanation-format__original">deny</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(Other definitions for refute that I've seen before include "Contradict, prove that statement is wrong" , "Prove something to be wrong" , "Disprove" , "Prove that statement or accusation is wrong" , "Prove to be incorrect" .)