Those working in the theatre producing Three Sisters, for instance (6)
I believe the answer is:
nurses
If this is the answer, I'm afraid I cannot explain it.
Can you help me to learn more?
(Other definitions for nurses that I've seen before include "Cares for during illness" , "Ministering angels" , "preserves" , "Matrons, midwives etc." , "Tends to needs of sick person" .)