Increase in loudness (9)
I believe the answer is:
crescendo
'increase in loudness' is the definition.
(I've seen this before)
This is all the clue.
(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.)" .)