Fruitless undertaking (5,6)
I believe the answer is:
fools errand
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">fruitless undertaking</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for fools errand that I've seen before include "Wild goose chase" , "Fruitless enterprise" , "Unsuccessful enterprise" , "Fruitless undertaking" , "Fruitless task" .)