Wavering note (7)
I believe the answer is:
tremolo
'wavering note' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for tremolo that I've seen before include "Paganini's effect" , "Quivering musical effect" , "Wavering note" , "A vibrating effect in music" , "Wavering effect in music" .)