Ethanol source (4)
I believe the answer is:
corn
'ethanol source' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for corn that I've seen before include "Hard growth on foot" , "Cereal; grain" , "Get drunk commonly" , "Hard thickening of the skin on a toe" , "Something unoriginal - cereal plant" .)