Improvise vocally (4)
I believe the answer is:
scat
'improvise vocally' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for scat that I've seen before include "Jazz singing - animal droppings - silvery fish" , "Buzz off" , "Jazz-singing" , "leave quickly" , "One way of singing" .)