I was able to replace the glass in the drivers side mirror of my 2018 Clarity. I purchased the part from OEM PartSource (oempartsource.com). They had to order it from Honda, but it was a genuine OEM original part with the electric defog heated back. Cost with shipping and tax was only $45 and change. I believe the part you need is: Mirror Sub-Assembly, R Part Number: 76203-TRV-A01. But be sure to give them your VIN so they can confirm. I found them very good to deal with.
The only tricky part in the installation process was pulling off the actual glass assembly from the mounting ball. It's very tight, and you have to really pull hard. (Also, remove the two wires from the connector tabs on the back of the glass.) Then pressing on the new mirror was a little scary. I was afraid I'd break the glass by pushing so hard, but I pressed only on the center right opposite to where the socket is located and it fortunately it popped on. Finally, you'll have to adjust the mirror manually to get it in sync with the electric positioning gears. You'll see what I mean, but with a little study and patience you'll get it adjusted.
Don't know what the dealer would charge for this but I'd suggest that you go ahead and get the part and give it a try. If you aren't successful or unsure, then just take it to your dealer for installation (or better, your favorite mechanic who will probably install it for much less!). Good luck.