A number swallowed up we hear (5)

I believe the answer is:

eight

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'a number' is the definition.
(I know that eight is a magic number)

I don't understand the remainder of the clue.

(Other definitions for eight that I've seen before include "Pints in gallon" , "Team involved in row" , "Perfect cube" , "Set of rowers" , "Oxygen's atomic number" .)

I've seen this clue in The Globe and Mail.

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