Rotten shade of red? (4)
I believe the answer is:
rust
'shade of red?' is the definition.
(I've seen this in another clue)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for rust that I've seen before include "Reddish-brown coating caused by oxidation" , "Oxidised iron" , "Reddish-brown coating formed on metal by moisture" , "Fungal plant disease" , "Red or brown oxide coating on iron or steel" .)