Lack of significance (12)
I believe the answer is:
unimportance
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">lack of significance</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 unimportance that I've seen before include "Lack of significance" , "Insignificance" .)