Son leaving smooth vegetable (4)
I believe the answer is:
leek
'vegetable' is the definition.
(I know that leek is a type of vegetable)
'son leaving smooth' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'leaving' suggests deleting specific letters.
'smooth' becomes 'sleek' (sleek means smooth or glossy).
'sleek' with 's' removed is 'LEEK'.
(Other definitions for leek that I've seen before include "Plant (onion relative) eaten as a vegetable" , "Plant related to the onion, pair with potato for soup" , "Green and white vegetable" , "Cylindrical vegetable" , "English town" .)