Omit to collect large threshing implement (5)
I believe the answer is:
flail
'threshing implement' is the definition.
(I've seen this before)
'omit to collect large' is the wordplay.
'omit' becomes 'fail' (failing is a kind of omiting).
'to collect' means one lot of letters goes inside another.
'large' becomes 'l'.
'fail' going around 'l' is 'FLAIL'.
(Other definitions for flail that I've seen before include "Thresher; wave helplessly about" , "Weapon; swing wildly" , "Grain thresher" , "Threshing device" , "Wave about wildly" .)