Thank you for contacting Telerik support team.
You have to add the following line of code:
this.get_itemsSelector().get_itemSelector()._selectorSearchBox.get_searchBox()._minCharacters = 1;
This code changes the minimum characters needed to activate the filtering in SearchBox. My example sets it to 1.
There is a PITS issue regarding clearing of search filter. You can vote for it. Here is a link:
Hope that this is helpful.Regards,