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In order to allow enduser delete content of a field (container also) you must allow entry on that field. Then, in browse mode, the enduser can use the Edit menu to Clear content. If you don't want allowing manual entry, create a button with the Clear Editing action, that target the field.
Set Field [Table::ContainerField ; "" ]
Will also clear the field in a script. Unlike Clear, it will work even if browse mode access is not permitted and even if the field is not even present on the current layout.