Window that opens in theatre (6)
I believe the answer is:
lancet
I believe this clue is a double definition.
'window' is the first definition.
(lancet window is a kind of window)
'that opens in theatre' is the second definition.
(blade used to open bodies in an operating theatre)
(Other definitions for lancet that I've seen before include "Surgical tool" , "A surgeon's two-edged knife" , "Pointed two-edged knife used by surgeon" , "Medical journal; window opening" , "Surgeon's instrument and journal" .)