Cryptic crossword clue
Coot up in the air, flying from such pollution 14

The most common and most likely answer to this crossword clue is the 14 letter word EUTROPHICATION

I've seen this crossword clue in Financial Times
E
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Buddy explains!

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

EUTROPHICATION noun
  1. The process of becoming eutrophic; the ecosystem's response to the addition of artificial or natural nutrients, mainly phosphates, through detergents, fertilizers, or sewage, to an aquatic system.

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