Null and void (7)
I believe the answer is:
invalid
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">null and void</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for invalid that I've seen before include "Void; patient" , "Having no effect" , "to be in poor health" , "Disable" , "patient [found]" .)