Material that's dangerous to walk over backwards? (5)

I believe the answer is:

denim

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'material' is the definition.
(type of fabric)

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

(Other definitions for denim that I've seen before include "Cotton twill fabric" , "Tough cotton fabric" , "Material for casual trousers or jackets" , "Fabric used for jeans" , "Twill cloth" .)

I've seen this clue in The Times.

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