Unfortunately, there is no way to clear a student's attempt for a test.  Canvas was considering adding this feature but ultimately chose not to (see Delete Quiz Attempts for more information).  Canvas emphasizes a focus on retaining records and maintaining historical data just in case those records are required for later review.  The current workaround for this is to give the student an additional attempt on the test.  This allows you to retain the record of the student's initial attempt in addition to their second attempt.

Further resources

• Once I publish my quiz, how can I give my students extra attempts?