The most common and most likely answer to this crossword clue is the 4 letter word ISMS
I've seen this crossword clue in The Guardian
I
S
M
S
Buddy explains!

The straight part
We know that "many philosophies" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case many philosophies = isms.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 ISMS mean?
ISMS noun- An ideology, system of thought, or practice that can be described by a word ending in -ism.
- Specifically, a form of discrimination, such as racism or sexism.