The crossword solution for the term 'Quartet Song Subject' is Adeline, which is a 7-letter word. Adeline is a popular song written by American singer-songwriter Ryan Adams and released in 2014. It is a love song about a woman named Adeline, and it has been covered by many artists since its release. The song has become a popular choice for quartets to perform, due to its catchy melody and heartfelt lyrics.