Crossword Genius

Rotten shade of red (4)

Ross

I believe the answer is:

rust

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'shade of red?' is the definition.
(I've seen this in another clue)

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

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(Other definitions for rust that I've seen before include "Reddish-brown coating caused by oxidation" , "Oxidised iron" , "Reddish-brown coating formed on metal by moisture" , "Fungal plant disease" , "Red or brown oxide coating on iron or steel" .)

I've seen this clue in the Evening Standard.
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