Let me guess, that you want to show a list of client by filtering on City and you want to show the list of filtered client in a portal.
If this is the case then you can do this in below way. Create a global text field lets say gCity_xt. Create a value list on the basis of Client City field with all the values. Put that global field gCity_xt and attach that value list so that you can able to select a city form the list of available city of clients. Now define a relationship between same Client table to Client with gCity_xt = City. Show the portal based on this relationship. Now if you will filter any city from the list of city of gCity_xt it will filter the result and show in the portal.
You can follow this process if this is what you want. Please correct me if i am wrong.