Metal corrosion (4)
I believe the answer is:
'metal corrosion' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for rust that I've seen before include "Reddish oxide coating on iron" , "A fungus; a corrosion" , "Reddish-brown coating formed on metal by moisture" , "Reddish coating formed on iron through 11 across" , "Reddish-brown coating formed on metal by damp" .)