Unrehearsed concert put out, I am opening (9)
I believe the answer is:
impromptu
'unrehearsed' is the definition.
(I've seen this before)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for impromptu that I've seen before include "With little or no preparation or forethought" , "Ad-lib" , "unrehearsed piece" , "Without much planning" , "Piano piece" .)