Quartet unchanged by upsetting spell (4)
I believe the answer is:
abba
'quartet' is the definition.
(ABBA, pop band quartet)
I cannot understand how the remainder of the clue works.
(Other definitions for abba that I've seen before include "famous pop group" , "Pater's" , "Father (NT)" , "Father (used of God in New Testament) - Swedish band" , "Biblical father" .)