Vegetable is large – it's frightening (4)
I believe the answer is:
leek
'vegetable' is the definition.
(leek is a kind of vegetable)
'large it's frightening' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'it's frightening' becomes 'eek' (I can't justify this - if you can you should believe this answer much more).
'l'+'eek'='LEEK'
'is' is the link.
Can you help me to learn more?
(Other definitions for leek that I've seen before include "Plant (onion relative) eaten as a vegetable" , "Foodstuff" , "Plant related to the onion, pair with potato for soup" , "National emblem of Wales." , "One of the veggies" .)