Vegetable experts should know? (5)

I believe the answer is:

onion

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'vegetable' is the definition.
(onion is a vegetable)

I don't understand how the rest of the clue works.

(Other definitions for onion that I've seen before include "Bulb used in cooking" , "One may be set" , "Eye-watering vegetable" , "Head (slang); vegetable" , "Pungent edible bulb of the lily family" .)

I've seen this clue in The Sun.

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