It’s 11 am Saturday and I’ve all but finished yesterday’s Listener puzzle, which is called “Sine Qua Non”. It has one of the most bewildering preambles I remember seeing in a Listener puzzle:
Most clues contain a misprint of one letter in the definition. The correct letters, in clue order, reveal hints for deriving two versions of a question from the remaining clues. The penultimate element of the first version is one of four that share identical components, as described by a five-letter definition concealed in the grid. These four elements and another five-letter word must be highlighted to show key information relating to the question and an initial representation of the questioner.
One of the four elements, interpreted differently, indicates which letter of which word in each non-misprint clue contributes to another message. The action it describes must be applied to the letters of the five-letter definition (and their counterparts) to reveal a representation of that element which must be highlighted in full. Finally, the key information must be modified to provide a consistent rendition. All entries are words in both the initial and final grids.
However, as I solved the puzzle, the instructions became clear, little by little. I have filled in the grid, found the two versions of the question, found the five-letter definition (which is a new word to me, and a surprising one, both because of its odd meaning and because of the way it ties in with the rest of the puzzle), found the four elements, found the key information, found the second message in the clues, and applied the second message to the five-letter definition to reveal the representation of the element.
The only thing left is to figure out how to alter the key information to “provide a consistent rendition”. A difficulty here is that there seem to be quite a few plausible choices for what I should alter this information to, and fully 11 of them will lead to valid entries in the grid after the alteration.
One of those possibilities sort of leaps out as being an obvious choice, but that’s based on the, um, representation of the element rather than on the phrase “consistent rendition”. I haven’t figured out how to interpret that phrase yet, and until I do I can’t be sure that the obvious choice is the right choice.
Very ingenious puzzle.
Scratchings 
I don’t know whether you have the time or interest to solve other complicated variety cryptics, but the American master of the type is the NPL’s Ucaoimhu, and he’s collected most of his previous ones here (and on a couple of other pages linked from this one): http://www.math.uchicago.edu/~wald/concryptics.html. There are scores of them, and I recommend them highly.