Biological unit (4)
I believe the answer is:
cell
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">biological unit</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for cell that I've seen before include "Prisoner's accommodation" , "Dungeon" , "Unit of animal or plant structure" , "Basic biological unit" , "celebrated Latin" .)