In the theatre, it's incisive (7)
I believe the answer is:
scalpel
'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.
Can you help me to learn more?
(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" .)