We do not want to clear all aggregated grades, only the one single assessment.
The workshop is currently in the Assessment Phase, and we are using Moodle 3.0
There is a way, but it is not obvious (on the other hand, it is not a common task, too). Assessments can be removed via the Manual allocation screen.
Find the row with the student's submission. The instructor's assessment should be displayed in the left column, with an "x" icon to remove it (displayed next to Terri Teacher in the demo screenshot below). Workshop will ask for confirmation that you really want to unallocate the already graded assessment.
Thank you, that's exactly what we needed!