"Fernando" pop group (4)
I believe the answer is:
'fernando pop group' 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 "Group winning the 1974 Eurovision Song Contest in Brighton" , "Form of address to deity" , "Palindromic 70s pop group" , "Biblical pop" , "Father (NT)" .)