Cartridge not written on (5)

I believe the answer is:

blank

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'cartridge' is the definition.
(blank is a kind of cartridge)

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

(Other definitions for blank that I've seen before include "Bare; not marked" , "Vacant space" , "Not written upon" , "nothing" , "Bare; not decorated" .)

I've seen this clue in The Mirror.

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