Musical surge (9)
I believe the answer is:
crescendo
'musical surge' 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 "Conceders (anag.)" , "Gradual loudening" , "Growing louder (mus.)" , "Gradual increase in loudness, musically" , "climax" .)