One short month is less than a decade! (3)

I believe the answer is:

dec

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'one short month' is the definition.
(abbreviation for December; 'short' indicates a short form)

I cannot understand how the remainder of the clue works.

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(Other definitions for dec that I've seen before include "Ant's TV partner" , "Ant's Geordie mate on Saturday night" .)

I've seen this clue in the Evening Standard.

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