Sought-after principal - an impetuous person (7)
I believe the answer is:
hothead
'an impetuous person' is the definition.
(I've seen this before)
'sought-after principal' is the wordplay.
'sought-after' becomes 'hot' (I've seen this in another clue).
'principal' becomes 'head' (synonyms).
'hot'+'head'='HOTHEAD'
(Other definitions for hothead that I've seen before include "maniac" , "Madcap" , "Impetuous irresponsible person" , "Person liable to get reckless impulses" , "An impetuous person with warm cranium" .)