I resolved configuring DHCP snooping on the switch.
Now the switch learn the client IP address in the CLI and it sends accounting Framed-IP address to the ClearPass as well:
SW(config)# sh port-access clients
Port Access Client Status
Port Client Name MAC Address IP Address User Role Type VLAN
----- ------------- ------------- --------------- ----------------- ----- -------------------------------------------------------
11 6c3be5f73105 6c3be5-f73105 10.90.26.204 GUEST MAC 148