Think the x idea is probably right, tying up 47a, 54a and 66d. Thank you Arran and Stevie. Not 100% satisfactory though feel we are just guessing at the answers.
Another thing I don't get is the reference to 'annual compound interest' in the preamble. And what's topical about ten anyway?? Can anyone enlighten me? Is the fact there are 17 clue-less answers meant to be relevant to 2017?