Improvise siren (4)
I believe the answer is:
vamp
I believe this clue is a double definition.
'improvise' is the first definition.
(I've seen this in another clue)
'siren' is the second definition.
(I know that siren can be written as vamp)
(Other definitions for vamp that I've seen before include "Seductive woman - improvised accompaniment" , "Femme fatale" , "Predatory woman" , "sexy woman" , "Unsophisticated musical improvisation" .)