Spontaneous — unrehearsed (9)
I believe the answer is:
impromptu
This clue is a double definition.
'spontaneous' is the first definition.
(I've seen this before)
'unrehearsed' is the second definition.
(impromptu means improvised or unplanned)
(Other definitions for impromptu that I've seen before include "Spontaneous" , "Off-the-cuff" , "Unplanned; unrehearsed" , "Ad hoc: spontaneous-sounding musical piece mastered by Schubert" , "Without rehearsal or preparation" .)