Oath he’d let loose — excitable guy (7)
I believe the answer is:
hothead
'excitable guy' is the definition.
(a hothead is an excitable person)
'oath he'd let loose' is the wordplay.
'let loose' indicates an anagram.
'oath'+'hed'='oathhed'
'oathhed' is an anagram of 'HOTHEAD'.
(Other definitions for hothead that I've seen before include "Harry Percy was one" , "Daredevil" , "maniac" , "Excitable person" , "Impetuous irresponsible person" .)