Hmm, I seem to have two mutually exclusive answers to 21ac and 18d.
Either I've got one wrong or there's been a cock-up.
21ac Drink - one round (3)
17d Correct again... (2-4)
18d ... without it? Waster (5)
I can't fully explain my answer for 18d which involves dropping an "e" but it fits the clue otherwise.