For an opening in the theatre, one is incisive (7)

I believe the answer is:

scalpel

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'for' is the definition.
Both the definition and answer are singular nouns.
Maybe there's a link between them I don't understand?

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for scalpel that I've seen before include "Operator may use this" , "Small surgical knife" , "Dissecting knife" , "Medical knife" , "Surgeon's tool" .)

I've seen this clue in the Evening Standard.

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