A rotten brownish colour (4)

I believe the answer is:

rust

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'a rotten brownish colour' is the definition.
Both the answer and definition are adjectives. Maybe you can see an association between them that I can't see?

This is all the clue.

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

I've seen this clue in the Evening Standard.

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