Can I use Xbox ones WiFi to teather to Internetless pc?

You could try getting another cord and just wiring the PC to the router as well.

No, sorry, not in any configuration that I know of. The Xbox has WiFi direct, but that's a peer to peer connection, and not for sharing a connection. On the PC you have access to the settings on the NIC to make a "bridge" connection, but as we do not have
access to the settings on the Xbox Console, you can't configure it to allow that bridge. Sorry.

Though if it's something you'd like to see, please post your idea to
feedback.xbox.com I don't think there are an physical limitations as to why it wouldn't be possible, but they'd have to build that into the system.

Your PC would have to have a WiFi hardware that is compatible of being turned into an access point.

If you have Windows 10 installed on your PC, simply open the settings, by clicking the start menu, and then clicking on the gear/cogwheel symbol on the lower left.

In the settings window, go to the network settings.

On the left choose the "Mobile Hotspot" category and enable the mobile hotspot.

You can also set a specific name and Passphrase for your Hotspot. Then you can try if your console sees that hotspot, and you can then use your PC's LAN connection to provide Internet to the console via WiFi.

Alternatively, open up a command prompt as administrator.

Enter the following command:

netsh wlan set hostednetwork mode=allow ssid=NAME key=PASSWORD

Replace NAME with the desired name for your WiFi Hotspot and PASSWORD for the Passphrase you want to use to encrypt your WiFi.

Be aware, that you can not use your WiFi to connect to the internet at the same time.

Your PC must have a LAN connection to the internet to be able to provide WiFi Hotspot functions.

If you want the Xbox to provide Internet to a PC, then this won't work. The Xbox can not provide WiFi Hotspot functions.

as far as i know you cant use the xbox wifi to share to the pc.

If your pc has an internet connection you can either connect this via ethernet to your xbox or you can use the wifi hotspot option on your pc to share its internet connection directly to your console so you can go online.

I don't think you can do that with the Xbox One. It has WiFi direct, but that's for connecting to the controllers.