http://purl.org/tcl/home/man/tcl8.4/TkCmd/clipboard.htm
A pointer or help discussing X vs Mac vs Windows clipboard processing - particularly the rather complicated X model - seems in order.
Perhaps a pointer to nice cross-platform tool to give a Tk view of the clipboards to which a user has access, with auto-saving and loading of the contents, would be a nice example?
Features of a Tk-based user interface to their system clipboards
See also Primary Transfer vs. the Clipboard.
Tk syntax help - Arts and crafts of Tcl-Tk programming - Category Command