Cut or knock down (4)
I believe the answer is:
fell
I believe this clue is a double definition.
'cut' is the first definition.
(felling is a kind of cutting)
'knock down' is the second definition.
(I've seen this before)
(Other definitions for fell that I've seen before include "Collapsed; hill" , "Came down - cut down" , "Cut down (tree), dropped" , "'I do not like thee Doctor ....'" , "Lake District hill" .)