Window opener in hospital (6)
I believe the answer is:
lancet
This clue is a double definition.
'window' is the first definition.
(lancet window is a kind of window)
'opener in hospital' is the second definition.
(blade used to open up a body)
(Other definitions for lancet that I've seen before include "Surgical knife with a pointed double-edged blade: pointed Gothic arch" , "A surgeon's two-edged knife" , "Surgeon's instrument and journal" , "Medical journal; window opening" , "Theatrical knife" .)