What happens if you create a script with just 'Clear' (so you don't specify the field), and make it the first script displayed under the Scripts menu? Then when you click in a field and use Cmd + 1 it will clear.
You may want to build a check into the script to avoid misclick, and to remind you what field you are about to clear...
In addition to Sorbsbuster's suggestion, you also have a couple of other options:
• Use cmd-A first followed by the delete key
• modify the fields in Layout mode to select the entire field when entered (Format>Field Control>Behavior)