Wavering musical effect (7)
I believe the answer is:
tremolo
'wavering musical effect' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for tremolo that I've seen before include "Musical wavering effect" , "Musical shake" , "Quivering musical effect" , "Vocal vibrato -- lot more (anag)" , "Quivering effect (in music)" .)