Need help with the following clues please.
21A: For the snooty, it's port out starboard home, in short ??S? (4)
28A: This clue has ANCIENT BUILDING printed in a circle ?O?O? ????R (5-5)
17D: ...spans most unusual flashes in time ... ?N???H??? (9)
27D: A sequel, perhaps to Cats ... as in cata-mauls P??? (4)
Thanks in advance.