Crosswords are generally filled in in all upper case, so I think "K" is valid. For "bust down reason", would this be something to do with not wearing a bra? I can't think of a 9-letter way of expressing that though.
Boolbar - you are correct on this one. But the reason I dislike it is because (a) it's ambiguous - the answer could just as easily be bookend; (b) there's no definition, purely a device; and (c) even as a device it's flawed because it's not indicated (e.g. with the phrase "to a setter"). These "inverse clues" are becoming more prevalent, but I don't think they're big or clever.
[Raak] That is true. You can make up several answers that match some of these clues - like O (4,6) = Come second. My 'v' is much the same, but is a famous schoolboy joke.