Core vegetable (6)
I believe the answer is:
marrow
I believe this clue is a double definition.
'core' is the first definition.
(I know that core can be written as marrow)
'vegetable' is the second definition.
(I know that marrow is a type of vegetable)
(Other definitions for marrow that I've seen before include "one of two" , "It fills the cavities of bones" , "essence" , "Oversized courgette" , "Fatty stuff in bones or large fruit" .)