As I think your question is about a stand-alone parameter, and not a table parameter, I just want to add the following example to Jarno's answer (which should be fine if you are working with table parameters):

In case of a stand-alone parameter you'll want to add the following action too. Otherwise, you won't see the result of the clear action:

<Action id="899">
<Name>ClearDisplayParameter</Name>
<On id="800">parameter</On>
<Type>clear on display</Type>
</Action>