Cmd-K is known to clear the static stdout in terminal.app and iTerm which doesn't work well at all with tmux. However, with the below key binding in iTerm2 you can clear the scroll back buffer per-pane.
In tmux.conf set the following binding
bind C-k send-keys -R \; clear-historyIn iTerm2 preferences, in the Keys tab add a keybinding like the following.
Caveats: This will override Cmd-K in iTerm2 for all non-tmux windows as well. However, if you work in tmux often, it might just be worth it.
I use this to prevent things from getting wonky when I reflexively hit cmd-k when in a program that doesn't share the its buffer with tmux:
It wraps the screen-clearing command with a check to see if you're currently in alternate screen mode, and does nothing if you are.