Single entity (4)
I believe the answer is:
unit
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">single entity</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 "Individual component" , "Single item, maybe part of a set" , "Individual thing as part of a whole" , "Small military detachment" , "Element or part" .)