There are two grammar issues here: 1. Relatives You can use either 'that' or 'which' as the relative pronoun here, and you can even miss it out entirely. I listened to a song about Green Elephant. I listened to the song about Green Elephant. 2. The use of 'a' or 'the' depends on whether the listener knows about the song or not. If the listener doesn't know about this song, you would say 'I listened to a song about Green Elephant.' If the listener knows about the song, for example, if your friend lent you a CD of songs, and said to you 'Which song did you listen to?', you would answer 'I listened to the song about Green Elephant.'

I listened to a song that was about (a) Green Elephant. ... Although, I would write: I listened to a song about a green elephant.