Entrusted with old iron? (4)

I believe the answer is:

rust

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'old iron?' is the definition.
Both the answer and definition are adjectives. Maybe you can see a link between them that I don't see?

'entrusted with' is the wordplay.
'with' indicates the answer is hidden within the clue.
'RUST' is hidden within 'entrusted'.

Can you help me to learn more?

(Other definitions for rust that I've seen before include "Reddish-brown coating caused by oxidation" , "Reddish-brown oxide coating" , "Reddish-brown coating formed on metal by damp" , "Reddish-brown colour" , "Reddish oxide coating on iron" .)

I've seen this clue in The Mirror.

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