Fiery boss an impetuous person (7)
I believe the answer is:
hothead
'an impetuous person' is the definition.
(I've seen this before)
'fiery boss' is the wordplay.
'fiery' becomes 'hot' (similar in meaning).
'boss' becomes 'head' (synonyms).
'hot'+'head'='HOTHEAD'
(Other definitions for hothead that I've seen before include "Headstrong type" , "Excitable person" , "Tearaway" , "Impulsive person" , "Harry Percy was one" .)