Bulbous vegetable (5)
I believe the answer is:
onion
'bulbous vegetable' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for onion that I've seen before include "reason for tears" , "veggie" , "Shape of Russian dome" , "Head (slang); vegetable" , "Edible tear-jerker" .)