Spooner's son spotted source of tofu (7)

I believe the answer is:

soybean

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'tofu' is the 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 can't see?

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for soybean that I've seen before include "American vegetable" , "Vegetable source of protein" .)

I've seen this clue in the Sydney Morning Herald.

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