Increase in volume (9)
(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" .)
'increase in volume' is the definition.
(I've seen this before)
This is all the clue.
(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" .)