Datum (4)
I believe the answer is:
fact
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">datum</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 fact that I've seen before include "True item; reality" , "Undisputed truth" , "It's certainly not fiction" , "Actuality" , "Truth, reality" .)