Agnetha, Benny, Bjorn and Anni-Frid (4)
I believe the answer is:
'agnetha benny bjorn and anni-frid' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for abba that I've seen before include "Old group" , "camel-hair coat" , "Renowned Swedish foursome" , "Swedish artists" , "rhymes in Petrarchan sonnet" .)