Hmmm. Your question is a little ambiguous. I'm going to assume you mean that you want a small duration of the file on the timeline. Highlight the section you want, either by dragging the mouse across it or by using I and O to mark the in and out points. Press Ctrl-T to trim. Can't get any quicker than that.

If you want a new file containing just this section then make the selection as above and render to a new file with the 'render loop region only' option selected.