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In a well known case, either of identical twins (3)

Ross

I believe the answer is:

pea

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'in' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for pea that I've seen before include "Cause of a princess's insomnia?" , "Little green vegetable" , "It grows in a pod" , "It grows in 23 Down" , "garden plant" .)

I've seen this clue in the Evening Standard.
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