Complex sentence: According to Clarke the song was written "in about five minutes" and when the song made its mark in America Clarke had already left the band, but Clarke feels that "it wasn't unfortunate" on account of he wrote the song.
Split into simpler sentences: According to Clarke, the song was written "in about five minutes". When the song made its mark in America, Clarke had already left the band; but, Clarke feels that "it wasn't unfortunate", since he had written the song.