Decay found beneath automobile plant (6)
I believe the answer is:
carrot
'plant' is the definition.
(carrot is a kind of plant)
'decay found beneath automobile' is the wordplay.
'decay' becomes 'rot' (rot can mean to decay).
'found beneath' is a charade indicator (letters next to each other) (I've seen this in other clues).
'automobile' becomes 'car' (synonyms).
'rot' after 'car' is 'CARROT'.
(Other definitions for carrot that I've seen before include "veg" , "The vegetable to induce" , "Vegetable to entice the donkey" , "Pointed vegetable" , "The vegetable for persuasion" .)