Oxidize (4)
I believe the answer is:
rust
'oxidize' is the definition.
(I know that rust is a more specific form of the action oxidize)
This is all the clue.
(Other definitions for rust that I've seen before include "deteriorate" , "Reddish coating formed on iron through 11 across" , "Fungal plant disease" , "A fungus; a corrosion" , "Corrode" .)