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 can't see how one could define the other.
I cannot understand how the rest of the clue works.
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" .)