Revoke, invalidate (6)
I believe the answer is:
cancel
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">revoke</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >'<span class="explanation-format__original">invalidate</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ ></span>
(Other definitions for cancel that I've seen before include "Rescind, abolish" , "Annul or call off" , "Write off" , "Call off or invalidate" , "Delete" .)