A rotten red (4)

I believe the answer is:

rust

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'red' is the definition.
(I know that rust is a type of red)

I cannot understand how the rest of the clue works.

(Other definitions for rust that I've seen before include "Oxidised iron" , "Fungal disease of plants" , "Reddish-brown coating formed on metal by damp" , "Fungal plant disease" , "Reddish-brown coating caused by oxidation" .)

I've seen this clue in the Evening Standard.

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