18A Telescope tracks (6) S - A - - -. I've seen STAIRS suggested, but can't see how that works - I was wondering about STARRY.
17D Top one of 18 above (6) - - - C - S. The second letter is the fifth in 18A, so there is a physical connection, if not also a logical one, though the reference to 18 above may not be to the afore-mentioned clue.
Any help with getting the last eight letters in this puzzle would be greatly appreciated.