O (4,6)

Dunno 'bout Martha's. I got the HIJKLMNO one pretty much immediately when I saw it, but it was in the Cryptic Crosswords and How to Solve Them book as part of a special feature on weird clues, so my mind was already pointed in the right direction. One that I saw in a Grauniad crossword, and don't actually think is that great a clue (for reasons I'll explain later) is "K (7)".

[MF] That one is my favorite clue. :-) I was reading down through the game and was about to post it, until I saw that you had got there first. Although I think the clue should actually be
O? (4, 6).
And here's a clue mentioned in the Sandy Balfour book that I recently reviewed at Orange, which although not perfect, you have to admire:
Bust down reason? (9).

Stop putting damn clues and no answers! Some of us are thick, you know!

Would that be (Highlight to reveal hidden text) full circle ?

Round number.

How about open letter?

Can't get that one - not enough letters for Potassium...

[rab] If it's Rufus, this must be Kelvins - if it's Enigmatist, I haven't got a chance :o)
No-one's got mine yet, and I don't think the "?" helps at all. Like HIJKLMNO, there's no definition involved.
As for "Bust down reason", I know this and I wouldn't've got it in a million years. At least, not without some letters.

Is O (4,6) like v (6,2,7) ??

Oh and is K Weekend?

Boolbar] I thought the same - but wouldn't the clue be in lower case, if that were the case? [MF] I'm still flummoxed by the 'O' clue but have a hunch it's more to do with numbers than letters.

What, Middle of Kelvins or something? Sadly no. Well done on K though.
[Chalky] Yes and no. Unhelpfully.

The Weekend one reminds me of one we had on Cryptic Stations which was e This was Mile End - though I, like Chalky was put off by the capitalization - and was looking for a "head" or "top" answer.
[MF] I thought Raak had got O - you're saying not ?

Nuts - that "head" was supposed to be hidden.

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.

Maybe lactating?

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.

[Raak] There's a simple way of expressing bra in 9 letters it's brassière. But apart from the bust bit, I can't see why it should be the answer.

[Bb] Centre of Gravity, ha ha. No, mine isn't the start, end or middle of anything. Or the second. Or even "a letter in the word 'open'".
[Blob] I said not when I said "No-one's got mine yet".