Ah - dingbats. That poor relation. Puzzles for the masses. Well, at least it keeps a classy clue 'alive'. Incidentally, the craze for dingbats - something else that the .. erm .. estimable Daily Mail has to answer for? dingbat [U S slang]n. something whose name one has forgotten, or does not want to use: a foolish or eccentric person: a tramp: money. Adj. [Austr. & N Z coll.] daft, crazy. [Austr. & N Z coll.] the dingbats: delirium tremens. Such provenance says it all.
Not too long - the brevity of the answer (just five letters) meant it had to be some clever way of reading the clue, so I just read it to myself in several ways and got there in a minute or so. But that's not to say it might not have taken me a while - perhaps I've seen something similar before, dunno.
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).