Iron oxide (4)

I believe the answer is:

rust

'iron oxide' is the definition.
(I've seen this before)

This is the entire clue.

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

I've seen this clue in The Times and The Star Tribune.

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