Hi,
As noted before, and still in 6.5 RC. Sometimes the search term is not "cleared" when the "clear" button is pushed. The search term can still be erased manually.
Of note: I have the option "Search Term Carry Over" enabled with "blank" as the "hotkey." This means that the search term is not erased when moving from email to inbox to files to documents etc.
I can't be sure, but I believe the clear always works, except when I go from one tab search to another and then back. For example: Search term "Fred" in email, then move to calendar (searching for "Fred" - carried over) then moving back to email (Searching for "Fred" - carried over). Then hitting Clear, Fred is still stuck in the search term box, and can only be removed by manually erasing (select FRED and delete).
Hope this helps.