40 clues, but 52 entries, so presumably 12 un-clued entries. If all 52 entries are real words does that mean that the jumbled entries are, in fact, anagrams of the answers to their clues, or does a jumbled entry still count as a real word?
Finally over the line having made very heavy weather of this, cold solving almost every clue before I twigged what was going on.
I think the title basically alludes to the fact that as a 'normal' crossword this would be an impossible construction. That said it is a highly impressive puzzle and (joy of joys!) totally unambiguous in the finale.
I interpreted the title as a reference to a phrase which includes part of the construction’s name and one of the unclued words, and means to attempt an impossible task?