You would use the parameterized string. Here is an example returning the the dial-in static IP value:
%{Authorization:<AD-Source-Name>:<Attribute-Name}
Here's an example for mine:
%{Authorization:AD_timcappalli-com:DialIn-StaticIP}
Since this is not a "typical" attribute for you to use in ClearPass from an AD source, you'll need to define it in your AD authentication source (that's why it has a more user friendly name).
On the attaributes tab of your AD source, click the "Authentication" filter, and add an attribute at the bottom. The "Name" is the exact attribute name in AD (easiest way to find all the attribiutes is to look at an account in ADSI Edit). "Alias Name" is what will be displayed in ClearPass as the attribute name. Choose string in this case.
This attribute will now be available for that AD authorization source in ClearPass.