Vichyssoise vegetable (4)
I believe the answer is:
leek
'vichyssoise vegetable' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for leek that I've seen before include "Vegetable symbol of Wales" , "Eat the vegetable and keel over" , "what Fluellen wore in France?" , "Vegetable emblem of Wales" , "Vegetable related to the onion" .)