Gradual increase in loudness (9)
I believe the answer is:
crescendo
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">gradual increase in loudness</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for crescendo that I've seen before include "Gradual rise in volume" , "Gradually getting louder" , "In music, a gradual increase in loudness" , "increase in sound level" , "Gradually increasing sound" .)