Corrode, as iron (4)
I believe the answer is:
rust
'corrode as iron' 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 coating caused by oxidation" , "Corrode" , "Reddish-brown coating formed on metal by moisture" , "Reddish-brown oxide coating" , "Fungal plant disease" .)