Set the bound field Visible property to No. In the After Update event of the Combo box put code similar to this:
If Me.ComboBox = "OptionYouWant" Then
Me.BoundTextBox.Visible = True
Else
Me.BoundTextBox.Visible = False
End If
Be sure the option you want in the Combo box is the bound column or you will need to refer to the correct 'column' in the code above....