Quatrain rhyme scheme (4)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">quatrain rhyme scheme</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for abba that I've seen before include "famous pop group" , "Swedish artists" , "Palindromic 70s pop group" , "Scandinavian supergroup" , "Old group" .)