Traditional words before play interrupt each session (5,1,3)
I believe the answer is:
break a leg
'traditional words before play' is the definition.
'interrupt each session' is the wordplay.
'interrupt' becomes 'break' (breaking is a kind of interrupting).
'each' becomes 'a' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'session' becomes 'leg'.
'break'+'a'+'leg'='BREAK A LEG'
(Other definitions for break a leg that I've seen before include "Good luck in the theatre!" , "Actors' good luck" , "Require surgery on limb?" , "leading to member being in cast?" , "Thespian's good luck! (slang)" .)