Chemical that removes hard bit from leather (5)

I believe the answer is:

oxide

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'chemical' is the definition.
(I know that oxide is a type of chemical compound)

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for oxide that I've seen before include "Rust, for instance" , "Rust is iron -" , "Eg, rust" , "Compound of two elements, one with atomic number 8" , "Chemical compound" .)

I've seen this clue in The Guardian.

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