Unwanted garden plant (4)
I believe the answer is:
weed
'unwanted garden plant' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for weed that I've seen before include "Marijuana (slang)" , "nuisance in bed" , "Uncultivated plant" , "Useless plant - useless person" , "dope" .)