Not false (4)
I believe the answer is:
true
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">not false</span>' is the definition.<br/ ><span class="explanation-format__explanation">('true' is the opposite to 'false')</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for true that I've seen before include "Agreeing with fact or reality" , "Straight or correct" , "Exact, factual" , "Constant" , "Veracious" .)