Vegetable - in retrospect, it's an impossibility to eat one (5)
I believe the answer is:
onion
'vegetable' is the definition.
(onion is a kind of vegetable)
'in retrospect it's an impossibility to eat one' is the wordplay.
'in retrospect' says the letters should be written backwards.
'it's an impossibility' becomes 'nono' (I've seen this before).
'to eat' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'nono' enclosing 'i' is 'noino'.
'noino' written backwards gives 'ONION'.
(Other definitions for onion that I've seen before include "Head (slang); vegetable" , "Alliaceous plant" , "reason for tears" , "Edible tear-jerker" , "Tear-jerking bulb" .)