Not even one (4)
I believe the answer is:
none
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">not even one</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for none that I've seen before include "Zero" , "0?" , "What's left after taking all" , "Not one thing" , "office" .)