Traditional words before play interrupt each session (5,1,3)
I believe the answer is:
break a leg
Here is my best explanation:
'traditional words before play' is the definition.
'interrupt each session' is the subsidiary indication.
'interrupt' becomes 'break' (breaking is a kind of interrupting).
'each' becomes 'a' (I can't justify this - if you can you should give a lot more credence to this answer).
'session' becomes 'leg'.
(Other definitions for break a leg that I've seen before include "Good luck in the theatre!", "leading to member being in cast?", "Require surgery on limb?", "Do something that requires surgery", "Actors' good luck", "but not on the piste", "Good luck wish offered to actor in bleak rage", "Thespian's good luck! (slang)" and "good luck wish".)