Short play before the main performance (7,6)
I believe the answer is:
curtain raiser
'short play before the main performance' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for curtain raiser that I've seen before include "warm-up" , "Preliminary event" , "Preliminary play" , "Opening move" , "Introductory show" .)