Number of vixen dash off northwards? (8)

I believe the answer is:

fourteen

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'number' is the definition.
(fourteen is a number)

I don't understand how the rest of the clue works.

(Other definitions for fourteen that I've seen before include "XIV" , "Pounds in a stone?" , "Number of lines in a sonnet" , "Fen route (anag)" .)

I've seen this clue in The Telegraph.

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