Hi tsrow
Take care when importing and always do a backup before each import - as importing cannot be 'un-done'.
And yes you would think that if a field can be set by import, it can be unset by import. However this is MYOB we are talking about. MYOB does not allow you to clear a field by import. You can change a field value from one value to another, however you cannot update a field with an empty/blank value. The 'Use Desc. On Invoice' is an X to set, however if you leave this blank, it will not update the current item.
I'm afraid it is one by one to fix this. Or there is a MYOB specialist who can do this in bulk for a fee, however with only 1300 items, probably more cost-effective to do it manually in your spare time.
Regards
Gavin