METHOD 1:
Go to listentoyoutube.com and enter the video's url to convert the video to mp3. Then find a free programme that will edit the sound so you've got the part of the song you want as the ringtone.
METHOD 2:
Download the youtube video (either via a site such as keepvid or with a plugin or with RealVideoPlayer). Also download a free audio ripping programme. Then rip the audio from the video at the exact paart of the audio you want.