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