Thresh about (5)
I believe the answer is:
flail
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">thresh about</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 flail that I've seen before include "Threshing device" , "Thresher; wave helplessly about" , "Grain thresher" , "Threshing implement" , "Old tool for threshing grain" .)