If you are going to use Sum, define the calculation field in the client's table, not the fees table. It's syntax would be:
Sum ( Fees::FeeAmountField )
If must evaluate from the context of Clients in order to display the correct value.
You can also use a summary field defined om tje fees table to show the same total, but if you are editing fees in the portal to that table, the calculation field using sum will update more smoothly than the summary field from fees.