The most common and most likely answer to this crossword clue is the 15 letter word HUMANITARIANISM
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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.
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What does HUMANITARIANISM mean?
HUMANITARIANISM noun- Humanitarian philosophy or practice.