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).