Vehicle to decay in plant (6)
I believe the answer is:
carrot
'plant' is the definition.
(carrot is a kind of plant)
'vehicle to decay' is the wordplay.
'vehicle' becomes 'car' (car is a kind of vehicle).
'to decay' becomes 'rot' (rot can mean to decay).
'car'+'rot'='CARROT'
'in' is the link.
(Other definitions for carrot that I've seen before include "Vegetable - incentive" , "Obstacle in race" , "orangey colour" , "One prov. used to encourage" , "Incentive (but not the stick)" .)