Plant rot (7)
I believe the answer is:
rhubarb
I believe this is a double definition.
'plant' is the first definition.
(rhubarb plant is a kind of plant)
'rot' is the second definition.
(both can mean rubbish or nonsense)
(Other definitions for rhubarb that I've seen before include "Purgative stalk" , "making contribution to murmuring" , "rot" , "Plant grown for its stalks" , "Plant with edible leaf stalk" .)