Vegetable - something to be avoided after I tucked in, on reflection (5)
I believe the answer is:
onion
'vegetable' is the definition.
(onion is a vegetable)
'i tucked in on reflection' is the wordplay.
'tucked in' means one lot of letters goes inside another (I've seen this in other clues).
'reflection' becomes 'on' (I am not sure about this - if you are sure you should believe this answer much more).
'on'+'on'='onon'
'i' placed inside 'onon' is 'ONION'.
'something to be avoided after' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for onion that I've seen before include "Pungent vegetable" , "Alliaceous plant" , "Bulb used in cooking" , "Hot dog topping" , "Pungent edible bulb of the lily family" .)