Possibly royal ingredient of cocoa, we're told (3)
I believe the answer is:
oak
'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" .)