The ability is there. Not sure why you can't find it.
Anyway, just open up your App Menu, and start the "Music" app. Find the song you want to make into a ringtone, and press the ... menu button. Select "Set as Ringtone". Once you do that, it should ask you if you want to use it as a Phone Ringtone, a Contact Ringtone, or if you would like to Trim the Ringtone. Select Trim the Ringtone and trim it however you would like.
Hope that helps, though, I do realize I'm months late to this.