Improvise, in jazz (4)
I believe the answer is:
vamp
'improvise in jazz' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for vamp that I've seen before include "Blatantly try to attract a man" , "Female seducer" , "Seductive woman - improvised accompaniment" , "Improvise inartistically" , "(In music) repeat phrase (until ready)" .)