What is this song from Episode 53 of Hunter x Hunter? I want to find the music from Episode 53 (Yorknew City arc) starting at 6:51

It's an Asian-style song with what sounds like a Japanese musical instrument called a shamisen.
As RichF said, the song of episode 53, at 6:51 doesn't sound asian-style at all, but if that is the song that you want, it's called Dirge From The Dark Side.