How does the "text box" get updated in the first place?
One approach would be to insert code in the AfterUpdate event of the combo
box, and have that event "tell" the text box its new value (description),
with something like:
Me!txtMyTextBox = Me!cboMyComboBox.Column(1)
where Column(1) refers to the second field/column in the query that feeds
the combobox (zero-based counting).