Number serious, though first ignored (6)

I believe the answer is:

eighty

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'number' is the definition.
(eighty is a kind of whole number)

I can't explain the remainder of the clue.

(Other definitions for eighty that I've seen before include "22 [FOUR] scores" , "Cardinal" , "4 score" , "Figure" , "Around the World in - Days (Verne)" .)

I've seen this clue in The Guardian.

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