A model - but not a working one (4-2)

I believe the answer is:

mock-up

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'a model' is the definition.
(I know that mock-up is a type of model)

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

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(Other definitions for mock-up that I've seen before include "Full-scale model built for experimental purposes" , "Dummy model" , "Model for testing purposes - sounds derisory" , "Full-size working model" , "A model of something, sounds derisory" .)

I've seen this clue in The Globe and Mail.

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