Warm-up act (6)
I believe the answer is:
'warm-up act' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for opener that I've seen before include "(Team's) first batsman" , "Card player who starts the bidding" , "In cricket, a batsman who plays the first ball of an innings" , "number one or two?" , "First going in to bat" .)