In a well known case, either of identical twins (3)
I believe the answer is:
pea
'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" .)