Bores quit changing nickname (9)
I believe the answer is:
sobriquet
'nickname' is the definition.
(I know that nickname can be written as sobriquet)
'bores quit changing' is the wordplay.
'changing' indicates an anagram.
'bores'+'quit'='boresquit'
'boresquit' with letters rearranged gives 'SOBRIQUET'.
(Other definitions for sobriquet that I've seen before include "A different name" , "Person's nickname" , "Another handle" , "one's assumed" , "Nickname -- Boers quit (anag)" .)