Numbers upset after eating one vegetable (5)
I believe the answer is:
onion
'vegetable' is the definition.
(onion is a vegetable)
'numbers upset after eating one' is the wordplay.
'numbers' becomes 'no no' (no is short for number as in no. 1).
'upset' says the letters should be written in reverse.
'after eating' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'nono' backwards is 'onon'.
'onon' enclosing 'i' is 'ONION'.
(Other definitions for onion that I've seen before include "Strong tasting vegetable" , "Head (slang); vegetable" , "Bulb used in cooking" , "Pungent edible bulb of the lily family" , "veggie" .)