gitto, 18a starts with the obvious abbrev (although it would seem to disrupt 1d it really does belong there).
The American slang is common enough - but usually in a different context.
Nearly all done now. The only outstanding part of this puzzle for me is identifying the work by 1d referred to by the last four in set D. Set C is very clever in my opinion.
I think it's nigh on impossible to find if you look specifically at 1d (it wasn't in the long list I waded through) and the clue to it isn't that helpful if you don't already know it. However, the other obvious wiki route pays off pretty quickly.