Agnetha, Björn, Benny and Anni-Frid? (4)
I believe the answer is:
abba
'agnetha bjorn benny 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 "Coptic bishop's title; Swedish pop group" , "Palindromic Swedish pop group who sang ` `Waterloo' '" , "Renowned Swedish foursome" , "Group - father" , "Form of address to deity" .)