"Not even once!" (5)
I believe the answer is:
never
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">not even once</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for never that I've seen before include "On no account" , "Not in a month of Sundays" , "Strong negative" , "Not ever" , "when pigs fly" .)