Peremptory demand (9)
I believe the answer is:
ultimatum
'peremptory demand' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ultimatum that I've seen before include "Final demand or statement of terms" , "Final proposition or demand" , "Uncompromising demand" , "Final offer" , "Final statement of actions required of another" .)