While Tk doesn't build in a combobox widget [will this change in 8.4?], there's a wealth of implementations available. Bryan Oakley's is pure-Tcl, widely used, and maintained efficiently. BWidgets is a popular pure-Tcl widget set [in Batteries Included distributions?] with an excellent combobox. [... List others, including Donal Fellows', the Iwidgets one, ...]
[Point to the docs to these various comboboxs!!!]
[Explain what a combobox is.]
[Mention tk_optionMenu.]