Vichyssoise ingredient (4)
I believe the answer is:
leek
'vichyssoise ingredient' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for leek that I've seen before include "Staffs. town; vegetable" , "Staffordshire town?" , "Vegetable symbol of Wales" , "Bulb" , "Plant; Welsh emblem" .)