Musical opening (5)
I believe the answer is:
intro
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">musical opening</span>' is the definition.<br/ ><span class="explanation-format__explanation">(short for introduction)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for intro that I've seen before include "Instrumental part at start of song" , "First passage of play" , "Preliminary part" , "Jazz piece" , "Beginning (colloq.)" .)