Warm-up act (6)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">warm-up act</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)