Vegetable turned double-negative round Irish capital (5)
I believe the answer is:
onion
'vegetable' is the definition.
(onion is a vegetable)
'turned double-negative round irish capital' is the wordplay.
'turned' is a reversal indicator.
'double negative' becomes 'nono' (I've seen this in another clue).
'round' indicates putting letters inside.
'irish capital' becomes 'i' (I am not sure about this - if you are sure you should believe this answer much more).
'nono' placed around 'i' is 'noino'.
'noino' backwards is 'ONION'.
Can you help me to learn more?
(Other definitions for onion that I've seen before include "Bulb used in cooking" , "associated with 26 [TRIPE] ?" , "Pungent vegetable" , "Edible tear-jerker" , "Bulb used as a vegetable" .)