Weed killer (9)
I believe the answer is:
herbicide
'weed killer' is the definition.
(I know that weed killer can be written as herbicide)
This is all the clue.
(Other definitions for herbicide that I've seen before include "Vegetation destroyer" , "Substance toxic to plants" , "Chemical agent that destroys plants" , "Plant killer" , "Weedkiller" .)