"SOS” quartet (4)
I believe the answer is:
abba
'sos quartet' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for abba that I've seen before include "Palindromic 70s pop group" , "Old group" , "Biblical pop" , "Swedish pop band of the 1970s" , "rhyme scheme" .)