The most common and most likely answer to this crossword clue is the 12 letter word CONTAMINATED
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The straight part
We know that "polluted" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case polluted = contaminated.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 CONTAMINATED mean?
CONTAMINATED verb- To make something dangerous or toxic by introducing impurities or foreign matter.
- To soil, stain, corrupt, or infect by contact or association.
- To make unfit for use by the introduction of unwholesome or undesirable elements.
- To infect, often with bad objects
- Adulterated; impure