Dandelion or crabgrass (4)
I believe the answer is:
This clue is a double definition.
'dandelion' is the first definition.
(I've seen this in another clue)
'crabgrass' is the second definition.
(I've seen this in another clue)
(Other definitions for weed that I've seen before include "Plant growing where it is not wanted" , "Wild unwanted plant - tobacco?" , "Made water" , "gone!" , "Uncultivated plant" .)