Nullify, cancel out (6)
I believe the answer is:
negate
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">nullify</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know that nullify is a kind of negate)</span><br/ ><br/ >'<span class="explanation-format__original">cancel out</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ ></span>
(Other definitions for negate that I've seen before include "Cancel; invalidate" , "Disprove or nullify" , "Render null or disprove" , "Nullify - deny the existence of" , "disallow!" .)