Make sure the song files are not in protected format. "Get Info," "Options," and set the start and stop times where you want your little clip to be. Click "OK," then play the song to make sure the clip will be as you like it. Select the song, "Advanced," "Convert Selection to XXX." That will create a new file with just your clip, to use as you wish.
Then, go back to the full song and remove the start and stop so it will play normally.