In court, she's defensive (6)

I believe the answer is:

portia

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'in' is the definition.
Both the definition and answer are singular nouns.
Maybe you can see a link between them that I can't see?

I can't explain the remainder of the clue.

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(Other definitions for portia that I've seen before include "A Shakespearean heroine" , "Heroine of Shakespeare's ''The Merchant of Venice''" , "She judged Shylock (M. of V.)" , "Shakespearian legal eagle" , "Heiress of The Merchant of Venice" .)

I've seen this clue in the Evening Standard.

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