Reddish-brown coating found in crustaceans (4)

I believe the answer is:

rust

I'm a little stuck... Click here to teach me more about this clue!

'reddish-brown' is the definition.
(I've seen this in another clue)

'coating found in crustaceans' is the wordplay.
'RUST' is hidden within the letters.
However, I'm uncertain how this is indicated.

Can you help me to learn more?

(Other definitions for rust that I've seen before include "Corrode" , "Reddish-brown coating formed on metal by moisture" , "Reddish oxide coating on iron" , "A fungus; a corrosion" , "Oxidisation" .)

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