Increase in loudness (9)
I believe the answer is:
crescendo
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">increase in loudness</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 crescendo that I've seen before include "Gradual increase in loudness, musically" , "Gradually increasing sound" , "Increase of volume in music" , "Gradually getting louder" , "Growing louder (mus.)" .)