Define the selection as a loop and save it.

To indicate to Adobe Audition that the selection is a loop, choose View > Wave Properties, and click the Loop Info tab. Then, select Loop and choose any other options you want, such as the tempo and key for the loop.

Now, save the loop: Choose File > Save As, and choose WAV as the format. If you want to save the loop as a smaller file, choose the CEL format, which is an MP3-compressed format for loops.