Green and white vegetable (4)
I believe the answer is:
leek
'green and white vegetable' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for leek that I've seen before include "Cylindrical vegetable" , "Vegetable related to onions" , "Vegetable - Welsh emblem" , "Vegetable emblem of Wales" , "Vegetable - Welsh symbol" .)