Musical buildup (9)
I believe the answer is:
crescendo
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">musical buildup</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 crescendo that I've seen before include "Increasing noise" , "Intensify" , "with score building" , "rising from piano?" , "Peak of intensity" .)