Either of those identical Green twins! (3)
I believe the answer is:
pea
'green twins' is the definition.
The answer and definition can be both things that one consumes for nourishment as well as being singular nouns.
Perhaps there's an association between them I don't understand?
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for pea that I've seen before include "Seed eaten as vegetable" , "Leguminous seed" , "Tiny stone" , "Leguminous vegetable" , "Ape this plant awkwardly" .)