If it's a song you own, you can find the song and then use an app to clip it to make it the length (and portion of the song) you want it to be. Send that clip over to your phone (i.e. via Google Drive) and put it into the Ringtones folder.
However, if you are just streaming the song using Apple Music and don't actually own it, then it won't work. Even if you attempt to download that file to your iTunes on your PC, it's a protected song.