Expensive stuff in the window at GAP sells badly (5,5)

I believe the answer is:

plate glass

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'expensive stuff in the window' is the definition.
I can't tell whether this definition defines the answer.

'at gap sells badly' is the wordplay.
'badly' is an anagram indicator (letters in the wrong order).
'at'+'gap'+'sells'='atgapsells'
'atgapsells' with letters rearranged gives 'PLATE-GLASS'.

Can you help me to learn more?

(Other definitions for plate glass that I've seen before include "Material for eg shop windows" , "shop-front?" , "Window material" .)

I've seen this clue in The Independent.

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