Physical effort (8)
I believe the answer is:
exertion
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">physical effort</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 exertion that I've seen before include "Strenuous activity" , "Physical/mental effort" , "Physical or mental effort" , "Great effort" , "Tire oxen with heavy effort" .)