If the combobox style is set to 0 - dropdown combo you can put the following in the InteractiveChange() event to trim the selected item:

This.DisplayValue = alltrim(This.Text)

However, this will not work when the style is set to 2 - Dropdown list.

One solution would be to create 2 columns, one with the textbox display value, the other with the indented version (or vice versa):

BoundColumn = 1 && Whatever column holds textbox display value.
ColumnCount = 2
ColumnLines = .F.
ColumnWidths = 0,100