Lie, untruth (3)
I believe the answer is:
fib
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">lie</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know that fib is a type of lie)</span><br/ ><br/ >'<span class="explanation-format__original">untruth</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(fib is a kind of untruth)</span><br/ ><br/ ></span>
(Other definitions for fib that I've seen before include "A harmless white lie" , "Story" , "Little white lie" , "Trivial lie" , "One must be 20 [UNTRUE]" .)