Is there a way to clear student answers so a retake is possible without previous answers being visible?
I don’t see anything on the interface that would indicate that feature exists. It appears the best you might be able to do is simply unlock the assessment so the student can change their answers.
I’ll ask and report back here if there’s more than that, but I wouldn’t hold my breath. 
Frank
Hi all, if you’d like to clear out a student’s answers / responses to any assessment or survey, just unlock it for them again, and any subsequent attempt of that survey or assessment will overwrite their previous work. Hope this helps!
We learned today that you must also “Make Visible” again - if you’ve turned that off.
AND students must select Unsubmit at the end of the assessment to enable editing if they have previously submitted