Onion is a vegetable (4)
I believe the answer is:
bean
This might be a double definition.
'onion' is the first definition.
The definition and answer can be both things that one consumes for nourishment as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
'is a vegetable' is the second definition.
(I've seen this before)
Can you help me to learn more?
(Other definitions for bean that I've seen before include "Seed; head" , "popular character with kids" , "Kidney-shaped seed as for coffee" , "Seed of coffee plant" , "Fava, kidney or mung, say" .)