Flail (6)
I believe the answer is:
thresh
'flail' is the definition.
(I know that flail can be written as thresh)
This is all the clue.
(Other definitions for thresh that I've seen before include "Separate grain with flail" , "Beat grain or make violent movements" , "Separate corn from husks by beating" , "Beat seeds out of grain" , "Beat out grain from husks of corn" .)