Unit of work (3)
I believe the answer is:
erg
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">unit of work</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 erg that I've seen before include "Unit of measurement of work" , "Energy expended" , "shifting sand" , "Unit of work or energy" , "Unit of work; dunes area" .)