How to clear field values?

I see how I can CLEAR by using 'Delete all items from the given picklist/radio/checkboxes/multiselect'. How can I clear the value from other fields like Date, single line and others?

I am finding it difficult to use the show and hide command without being able to clear values on HIDE.

In my other example, I have a 3 fields Allergy, Severity and Allergen. Severity and Allergen are hidden on form load and shown when Allergy == 'Yes'. If you add info to the 2 fields and then go back and change the Allergy to 'No', the 2 other fields are hidden but data values are not deleted. As a result the database has incorrect data in these instances.