You can't revert a clip that has been replaced with an AE comp back to its original state. However, depending on how much work you've done in Premiere since replacing the clip, you can usually just use the Undo command and the clip will "come back."
If you've gone past that point too far, or closed your project, you can drag the original clip from the Project Panel to the AE comp in the sequence, hold down the Alt key, and release the mouse. This will replace the comp with the clip. You'll have to adjust its in and out points, but that will essentially get you back to square one.