Cut down highest vegetable (3)
I believe the answer is:
pea
'vegetable' is the definition.
(I've seen this before)
'cut down highest' is the wordplay.
'cut down' means to remove the last letter.
'highest' becomes 'peak' ('peak' can be a synonym of 'highest').
'peak' with its final letter taken off is 'PEA'.
(Other definitions for pea that I've seen before include "Edible green seed" , "Ape this plant awkwardly" , "food item" , "The plant is a strange ape" , "Spherical vegetable" .)