There isn't a way for students or instructors to clear answers to an activity and "start over from scratch." However, this isn't a problem!
Students: As long your instructor hasn't closed the assignment, any changes you make are saved automatically, and your instructor will always see the most recently saved version of your answers. If you would like to start over from scratch, simply delete all of your answers manually.
Instructors: When grading a student's response, click the three dots next to Save Progress and select Reset Student Response. This cannot be undone.
One exception to this are locked sections.