Down on borders of hill harvest vegetable (8)
I believe the answer is:
kohlrabi
'vegetable' is the definition.
(kohlrabi is a kind of vegetable)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for kohlrabi that I've seen before include "Variety of cabbage like a turnip" , "leaves to be consumed" , "vegetable" .)