Re livs' post, some way back, about the central clues:
I think the numbers in these clues refer to the position of letters in alphabets, rather than to other clues:
21a - ETA is the 7th letter of the Greek alphabet, which has 24 letters ("7 of 24")
17d & 18d - ARE sounds like the 18th letter of the English alphabet, and TEA like the 20th ("18 ... / ... or 20, say, of 26")
I guess the other parts of these clues then provide double definitions?