Cryptic crossword clue
Essentially toxic delta area turning into rust 9

The most common and most likely answer to this crossword clue is the 9 letter word OXIDATION

I've seen this crossword clue in Independent
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Buddy explains!

The straight part
We know that "rust" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rust = oxidation.
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 OXIDATION mean?

OXIDATION noun
  1. The combination of a substance with oxygen.
  2. A reaction in which the atoms of an element lose electrons and the oxidation state of the element increases.

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