tmux has no direct access to the X clipboard itself. Reading the clipboard with OSC 52 is possible but it is unfortunately not widely supported by terminal emulators (you can do it with `refresh-client -l` if the terminal emulator supports it).

Or you could read it with `xclip`, `xsel` or `pbpaste`, something like:

    bind -n MouseDown2Pane run "pbpaste|tmux loadb -" \; pasteb

From tmux 3.2, middle click will paste the top tmux paste buffer by default.