Such models are apt to go to the wall (3-2)

I believe the answer is:

pin-up

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'wall' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

I can't explain the rest of the clue.

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(Other definitions for pin-up that I've seen before include "Alluring poster" , "picture of heart-throb?" , "Attractive figure" , "Object of (sexual) admiration" , "Poster of an attractive person" .)

I've seen this clue in the Evening Standard.

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