Threshing instrument (5)
I believe the answer is:
flail
'threshing instrument' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for flail that I've seen before include "Threshing implement" , "Thresh about" , "Old tool for threshing grain" , "Threshing device" , "Grain thresher" .)