A number dry, waters finally drained out (6)

I believe the answer is:

thirty

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'a number' is the definition.
(thirty is a kind of number)

I cannot understand how the rest of the clue works.

(Other definitions for thirty that I've seen before include "Number of pieces of silver paid to Judas" , "Six times five" , "Roman number XXX" , "Number of across clues in this puzzle" , "Score and a half" .)

I've seen this clue in The Telegraph.

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