Cut off a weed (4)
I believe the answer is:
dock
This clue is a double definition.
'cut off' is the first definition.
(dock can mean to cut or remove)
'a weed' is the second definition.
(type of weed)
(Other definitions for dock that I've seen before include "Antidote to nettle stings" , "Where a prisoner is situated in court" , "land" , "Curtail - wharf" , "Boat mooringd" .)