Just select the song in iTunes, press Cmd+i (to get info) and under Options you can change the start time. From there, just change your import options to mp3 with a low(er) bit rate, and then (with the song still selected) under the Advanced menu select "Convert Selection to mp3." -This will create a separate audio file in mp3 format that you can use as a ring tone.
...Don't forget to change your song options and import preferences back to the originals!
Note: This will NOT work with songs purchased from the iTunes Store.