Impestuous person? He had to reform (7)
I believe the answer is:
hothead
'impestuous person? he had to reform' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he had to reform' is the wordplay.
'reform' indicates anagramming the letters (letters in a new form).
'he'+'had'+'to'='hehadto'
'hehadto' anagrammed gives 'HOTHEAD'.
Can you help me to learn more?
(Other definitions for hothead that I've seen before include "An impetuous person with warm cranium" , "Excitable person" , "Tearaway" , "Impulsive person" , "Impetuous irresponsible person" .)