Basically what the title says. My friend is the one who posted the clan desc/thread, but even though I am also a leader, I can only edit it in classic forum mode.

Modern forum doesn't allow me to edit the thread, so I have to keep changing it to modify the desc. Any ix for this?

try this http://forum.toribash.com/clan_thread_edit.php?clanid=YourClanIdHere

you can save it somewhere until they fix the edit button

Clans / Market / Shop / anything else not forum related isn't yet fully supported by new design (thus the beta in its name) - thus the possible issues. It's most likely due to custom legacy scripts used in Clans page, you'll need to switch to classic style to use it for now.