Thanks THEAMBLER for getting back so fast. Physics isn't my thing but I guess BARYON meets the 'something very small' requirement. But I still don't get the clue. Many thanks all the same
Many thanks AB for the clarity of your explanation. I'd raced through the other clues in no time at all...got a bit over-confident...and hadn't expected anything as obscure as this. Aaaah, the sin of vanity!