Dishy school principal, an impetuous person (7)
I believe the answer is:
hothead
'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).
'hot'+'head'='HOTHEAD'
(Other definitions for hothead that I've seen before include "Person liable to get reckless impulses" , "Daredevil" , "Quick-tempered person" , "Headstrong type" , "Fiery character" .)