Gradual increase in loudness (9)
I believe the answer is:
'gradual increase in loudness' is the definition.
(I've seen this before)
This is the entire clue.
(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" .)