Mustard family member (4)
I believe the answer is:
kale
'mustard family member' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for kale that I've seen before include "Cabbage in a lake" , "Vegetable" , "Hardy cabbage" , "Curly-leafed cabbage" , "Something green" .)