Bringing up the rear, as usual! Having pored over this week's Cryptic, I had to admit (53a and 48d) to a lack of knowledge of foreign beasties. Now, I am for some reason stuck just on 3d "Encouraging word, I hesitate to say, is something corrosive". Help, please!
On another tack completely, resorting to Google, I seem to have come up with NANILLIONTH for the "tiny portion" in 2d (based on the prefix "nano" - Greek, I think - meaning tiny. Looks like I'm the only solver who has! Bet all the others are right, though!