Vegetable in stock, a leek (4)
I believe the answer is:
kale
'vegetable' is the definition.
(I know that Kale is a type of Leaf vegetable)
'in stock a leek' is the wordplay.
'in' says the answer is hidden in the clue.
'KALE' is hidden in the letters of 'stock a leek'.
(Other definitions for kale that I've seen before include "Curly-leafed cabbage" , "(Curly?) cabbage" , "Type of cabbage" , "Cabbage in a lake" , "Vegetable" .)