Jazzy improvisation (4)
I believe the answer is:
scat
'jazzy improvisation' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for scat that I've seen before include "Buzz off" , "Leave; jazz singing" , "leave quickly" , "Animal droppings - small silvery fish" , "Type of jazz singing - be off!" .)