Vegetable essence (6)
I believe the answer is:
marrow
I believe this clue is a double definition.
'vegetable' is the first definition.
(marrow is a kind of vegetable)
'essence' is the second definition.
('marrow' can be a synonym of 'essence')
(Other definitions for marrow that I've seen before include "Matter in bones or a vegetable" , "Vegetable - tissue in bone" , "Fatty stuff in bones, or a vegetable gourd" , "Connective tissue in bones" , "essential part" .)