The most common and most likely answer to this crossword clue is the 6 plus 3 letter word TURNIP TOP
I've seen this crossword clue in The Times
T
U
R
N
I
P
T
O
P
More clues leading to the result TURNIP TOP
Buddy explains!

The straight part
We know that "Vegetable" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case vegetable = turnip top.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.