A number vamoose, or split (6)

I believe the answer is:

cleave

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'split' is the definition.
(I know that split can be written as cleave)

I cannot understand how the rest of the clue works.

(Other definitions for cleave that I've seen before include "Chop or stick to" , "Open" , "Fissure" , "Chop or split" , "Join or split" .)

I've seen this clue in the Evening Standard.

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