In Interfaces > Bridges you can define a new bridge and add interfaces to it. The go to Interface Assignments, assing an interface to the new bridge and enable it. No further settings are needed on the bridge interface.
But befor you have to ensure that there is no configuration on the vlan 10 interface. It has only to be enabled.
However, with this setting results in the vlan 10 going down, when WAN goes down. To avoid that you can move the IP settings from the WAN interface to the bridge.