Nullify or disprove (6)
I believe the answer is:
negate
<span class="explanation-format__ClueExplanation">I believe this 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 can be written as negate)</span><br/ ><br/ >'<span class="explanation-format__original">disprove</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 "Render ineffective" , "Countermand" , "Nullify - deny the existence of" , "Cancel; invalidate" , "Cause to have no effect" .)