Kay, one with the French cabbage (4)
I believe the answer is:
kale
'cabbage' is the definition.
(kale is a kind of cabbage)
'kay one with the french' is the wordplay.
'kay' becomes 'k'.
'one' becomes 'a'.
'with' says to put letters next to each other.
'the french' becomes 'le'.
'k'+'a'+'le'='KALE'
(Other definitions for kale that I've seen before include "Cabbage in a lake" , "Crinkled-leaf cabbage" , "Type of cabbage" , "Cabbage variety" , "Curly-leafed cabbage" .)