Since this puzzle has GWIT written all over it, and I'm not good at cold solving, my first instinct was not to bother.
Anyhow I've given it my best shot but have run into the following brick wall:
(1) I've solved 18A
(2) I've solved both 15D clues
(3) I've solved one of the 17A clues, but it's not consistent with the letters I have in the grid from (1) and (2).
Moral of story: always follow first instinct.