I'd never heard of 4 either. Wrote in the rather obvious first word but took ages to parse the second. Then had to google the putative solution. It's a witty clue, though.
Yes it made me smile. I was trying to find something Latin and failing! It was the first word I couldn't see until 1a came to me. I didn't know the 1a meaning either but makes sense once you see it. All done :-)
1a is a rare clue for which Yanks have the advantage. The first word is common on this side of the pond, but I've seldom (if ever) seen the second word added here.