Do one's utmost (6)
I believe the answer is:
strive
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">do one's utmost</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for strive that I've seen before include "Contend using rivets" , "Endeavour" , "Try hard, carry on conflict" , "Work hard - compete" , "Try hard to break rivets" .)