Crossword Genius

In the theatre, it's incisive (7)

Ross

I believe the answer is:

scalpel

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

I can't explain the remainder of the clue.

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(Other definitions for scalpel that I've seen before include "Surgical tool" , "Sharp cutting tool" , "Surgical implement" , "Small surgical knife" , "One making cuts in hospital" .)

I've seen this clue in the Evening Standard.
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