Requiring great exertion (9)
I believe the answer is:
strenuous
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">requiring great exertion</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 strenuous that I've seen before include "Hard" , "Physically demanding" , "Vigorous" , "Requiring great effort" , "Taxing" .)