Fiery principal an impetuous person (7)
I believe the answer is:
hothead
'an impetuous person' is the definition.
(I've seen this before)
'fiery principal' is the wordplay.
'fiery' becomes 'hot' (synonyms).
'principal' becomes 'head' (synonyms).
'hot'+'head'='HOTHEAD'
(Other definitions for hothead that I've seen before include "Madcap" , "Person liable to get reckless impulses" , "Quick-tempered person" , "Excitable person" , "Fiery character" .)