Invalidate, negate (5)
I believe the answer is:
annul
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">invalidate</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >'<span class="explanation-format__original">negate</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 annul that I've seen before include "scratch" , "Repeal, rescind" , "Cancel, revoke" , "Scrap" , "Quash" .)