Crossword Genius

Quartet unchanged by upsetting spell (4)

Ross

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" .)

I've seen this clue in The Guardian.
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