King fish served up with vegetable (4)
I believe the answer is:
leek
'vegetable' is the definition.
(leek is a kind of vegetable)
'king fish served up' is the wordplay.
'king' becomes 'k' (chess abbreviation).
'fish' becomes 'eel' (eel is a type of fish).
'served up' shows that the letters should be reversed in order (in a down clue, letters go up).
'k'+'eel'='keel'
'keel' reversed gives 'LEEK'.
'with' is the link.
(Other definitions for leek that I've seen before include "Eat the vegetable and keel over" , "Bulb" , "Welsh national symbol" , "what Fluellen wore in France?" , "Vegetable - Welsh emblem" .)