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!!!]
Doc for incr Widgets combobox can be found in http://purl.org/tcl/home/man/iwidgets3.0/combobox.n.html
Try this one: http://incrtcl.sourceforge.net/iwidgets/iwidgets/combobox.html
[Explain what a combobox is.]
[Mention tk_optionMenu.]