Reality; certainty (4)
I believe the answer is:
fact
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">reality</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know that fact is a type of reality)</span><br/ ><br/ >'<span class="explanation-format__original">certainty</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ ></span>
(Other definitions for fact that I've seen before include "Not fake news!" , "An indisputable truth" , "copyright protection agency?" , "given" , "True statement" .)