Pointed window essential in theatre (6)
I believe the answer is:
lancet
This clue is a double definition.
'pointed window' is the first definition.
(type of window)
'essential in theatre' is the second definition.
(essential item in an operating theatre)
(Other definitions for lancet that I've seen before include "Surgical instrument" , "Sharp instrument" , "Pointed two-edged knife used by surgeon" , "Surgical tool" , "Surgeon's tool" .)