Oxidation (4)
I believe the answer is:
rust
'oxidation' is the definition.
(I know that rust is a type of oxidation)
This is the entire clue.
(Other definitions for rust that I've seen before include "Reddish oxide coating on iron" , "Reddish-brown colour" , "Reddish-brown coating caused by oxidation" , "Reddish coating formed on iron through 11 across" , "A fungus; a corrosion" .)