Single element (4)
I believe the answer is:
unit
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">single element</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for unit that I've seen before include "One part of a whole" , "- - trust" , "A single self-contained item" , "Individual thing as part of a whole" , "Self-contained part" .)