Increase in volume (7)
(Other definitions for amplify that I've seen before include "Make fuller, increase the strength of" , "Increase (a signal)" , "Expand" , "Add strength or details to" , "Make (signal) bigger" .)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">increase in volume</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 "Increasing noise" , "(Of music) gradually increasing in loudness" , "Gradually getting louder" , "Gradual increase in sound" , "Building up of sound" .)