Impetuous, spontaneous (9)
I believe the answer is:
impulsive
I believe this is a double definition.
'impetuous' is the first definition.
(I know that impetuous can be written as impulsive)
'spontaneous' is the second definition.
(similar in meaning)
(Other definitions for impulsive that I've seen before include "Done without forethought" , "Rash" , "Spontaneous" , "Acting without forethought" , "responding promptly" .)