The most common and most likely answer to this crossword clue is the 8 letter word BEETROOT
I've seen this crossword clue in The Guardian
B
E
E
T
R
O
O
T
More clues leading to the result BEETROOT
- Better perhaps putting in duck's eggs as part of the salad
- Defeat, we hear, England’s captain
- Vegetable better cooked — nothing old should be swallowed
- Soundly thrashed, English batsman visibly embarrassed
- 9 Buzzer and horn sound about right for Russian course requirement
- Time engaged in booze – very much turning purple
- Insect with steady pace consuming old vegetable
- Maggot gets round gnarled tree and round vegetable
- Excel, having way, it’s said, to produce vegetable
- Gamble to go bad with hearts doubled, a cause of red hands when dicing?
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 = beetroot.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.
What does BEETROOT mean?
BEETROOT noun- Beta vulgaris, a plant with a swollen root which is eaten or used to make sugar.
- A beetroot, a swollen root of such a plant.
- (usually uncountable) The edible part of the root of a beet plant, raw or prepared.
- To turn a bright red or purple colour.