Vegetable rubbish under vehicle (6)
I believe the answer is:
carrot
'vegetable' is the definition.
(I've seen this before)
'rubbish under vehicle' is the wordplay.
'rubbish' becomes 'rot' (I've seen this before).
'under' says to put letters next to each other.
'vehicle' becomes 'car' (car is a kind of vehicle).
'rot' put after 'car' is 'CARROT'.
(Other definitions for carrot that I've seen before include "Red vegetable used as inducement" , "Obstacle in race" , "veg" , "The vegetable for persuasion" , "Rewarding vegetable?" .)