Beginning of play being performed: start filming! (6)
I believe the answer is:
action
'start filming' is the definition.
(lights, camera, action!)
'beginning of play being performed' is the wordplay.
'beginning of play' becomes 'Act I' (first act of a play).
'being performed' becomes 'on' (eg a play may be 'on' at a particular theatre).
'acti'+'on'='ACTION'
(Other definitions for action that I've seen before include "SECOND PART OF 15" , "exploit" , "instruction from director" , "Deed; armed conflict" , "Deed, maybe battle" .)