Dishy school principal, an impetuous person (7)
I believe the answer is:
'an impetuous person' is the definition.
(I've seen this before)
'dishy school principal' is the wordplay.
'dishy' becomes 'hot' (dishy can mean attractive).
'school principal' becomes 'head' (synonyms).
(Other definitions for hothead that I've seen before include "Person liable to get reckless impulses" , "Daredevil" , "Quick-tempered person" , "Headstrong type" , "Fiery character" .)