Result of oxidation (4)
I believe the answer is:
rust
'result of oxidation' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rust that I've seen before include "Reddish-brown colour" , "Reddish-brown oxide coating" , "Oxidised iron" , "Corrode" , "Reddish oxide coating on iron" .)