The most common and most likely answer to this crossword clue is the 8 letter word CORRODED
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The straight part
We know that "rusty" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rusty = corroded.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does CORRODED mean?
CORRODED verb- To eat away bit by bit; to wear away or diminish by gradually separating or destroying small particles of, as by action of a strong acid or a caustic alkali.
- To consume; to wear away; to prey upon; to impair.
- To have corrosive action; to be subject to corrosion.
- Damaged by corrosion.