Corrode (4)
I believe the answer is:
rust
'corrode' is the definition.
(I know that corrode can be written as rust)
This is all the clue.
(Other definitions for rust that I've seen before include "Reddish coating formed on iron through 11 across" , "Fungal plant disease" , "Reddish-brown coating formed on iron by moisture" , "Reddish-brown coating formed on metal by moisture" , "A fungus; a corrosion" .)