The most common and most likely answer to this crossword clue is the 14 letter word EXISTENTIALISM
I've seen this crossword clue in The Guardian
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More clues leading to the result EXISTENTIALISM
Buddy explains!
The straight part
We know that "philosophy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case philosophy = existentialism.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 EXISTENTIALISM mean?
EXISTENTIALISM noun- (not countable) A twentieth-century philosophical movement emphasizing the uniqueness of each human existence in freely making its self-defining choices.
- The philosophical views of a particular thinker associated with the existentialist movement.