Fungus that damages iron (4)
I believe the answer is:
rust
This clue is a double definition.
'fungus' is the first definition.
(I know that rust is a type of fungus)
'that damages iron' is the second definition.
(iron can be affected by rust)
(Other definitions for rust that I've seen before include "Reddish-brown coating formed on metal by damp" , "Oxidised iron" , "Fungal plant disease" , "Reddish-brown coating formed on metal by moisture" , "Oxidisation" .)