The most common and most likely answer to this crossword clue is the 15 letter word CONSERVATIONIST
I've seen this crossword clue in The Guardian
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More clues leading to the result CONSERVATIONIST
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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 CONSERVATIONIST mean?
CONSERVATIONIST noun- A person who maintains natural areas or protects threatened species.
- Someone who advocates for these actions.