Delete item in grid

I have a 1 column x 6 row grid. Each cell is populated from a drop-down list.
If you double-click in a cell, then you get the drop-down list and can pick one of the items. So far, so good. Not all rows are required; if someone populates the cell in the first row, then clicks into the 2nd row cell and accidentally clicks on the drop down list, now they have populated that cell. How can they clear that cell? I am thinking of adding a <delete> item to the drop-down list, and if they select that then I could clear the value from the grid, but I was hoping to find a "native" way to do it. Pressing ESC will clear the drop-down selection the first time (the cell reverts to null), but if you click out of the cell and then back in there's no way to clear it, and pressing ESC there just closes the whole form.