Exertion (6)
I believe the answer is:
effort
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">exertion</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I know that exertion can be written as effort)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for effort that I've seen before include "'Endeavour, exertion (6)'" , "Serious industry - attempt" , "For eft (anag.)" , "Attempt - hard work" , "Struggle" .)