See optionMenu.
Note: Unlike most Tk widgets, this widget's name (optionMenu) is not the same as its creation command (tk_optionMenu). The code for this command is found in Tk's optMenu.tcl library file.
Do you want "an event binding for selecting a value from tk_optionMenu"? Tom Wilkason pointed out in a comp.lang.tcl thread that "The most common approach is to have a trace on the linked variable."
Tk syntax help - Arts and crafts of Tcl-Tk programming Category Command