Iron oxide (4)
I believe the answer is:
rust
'iron oxide' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for rust that I've seen before include "Fungal disease of plants" , "Corrode" , "Reddish-brown coating caused by oxidation" , "Reddish-brown coating formed on metal by damp" , "Reddish-brown coating formed on iron by moisture" .)