Well that's a significant detail to leave out, and that's normal behavoir for any app. Just like when editing a Word document, you can't selectively undo a step you made 50 edits ago without also undoing all the changes you've made since then.
Instead, it sounds you're looking for a simple way to restore a clip to its original state. There are some tools and effects that you can do exactly that in PD, but unfortunately the Crop tool isn't one of those.
If you've previously placed that clip in the Media Room, it's very easy to drag a new copy onto the timeline and use that instead of the cropped clip. If you don't have an easy way of finding the original, go to the current project's Export folder and browse the "Croped Pictures" folder (yes, it's actually mispelled) to find a copy of the original with a (0) added to the end of the file name.
You can also find the location by right clicking on the clip on PD's timeline and choosing Properties. This is what you should see when looking at that folder with File Explorer: