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Possibly royal ingredient of cocoa, we're told (3)

Ross

I believe the answer is:

oak

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'possibly royal' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for oak that I've seen before include "Traditional British tree" , "Acorn tree" , "Hardwood tree" , "Hard durable wood of a deciduous tree" , "Grown acorn" .)

I've seen this clue in the Evening Standard.
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