Introductory musical piece (7)
I believe the answer is:
prelude
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">introductory musical piece</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for prelude that I've seen before include "Opening piece" , "Introductory piece of music" , "Introductory musical piece" , "Curtain-raiser" , "Introductory passage" .)