Using maximum effort (3-3)
I believe the answer is:
all-out
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">using maximum 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 all-out that I've seen before include "Whole-hearted (effort)" , "Using every effort - everybody exit" , "Nobody home, or using every possible speed" , "Using every resource" , "Using maximum energy - no one at home?" .)