Vegetable related to onions (4)
I believe the answer is:
leek
'vegetable related to onions' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for leek that I've seen before include "Onion's relative" , "Vegetable related to onions" , "One of the veggies" , "Plant (onion relative) eaten as a vegetable" , "what Fluellen wore in France?" .)