Unselect Single Choice Custom Field

No, unfortunately, you have to select another option to not clear it.

When people request the ability to unselect a single choice, I recommend using a drop-down custom field instead of a single choice and adding a ";" at the beginning of the string.  The semicolon will add a blank as the first option.

The only warning I have is if you are using an old version of WHD, the blank field will count as a choice when using a required field.  SolarWinds fixed this issue in the last few versions.  I don't remember at what version SolarWinds resolved the issue.