The most common and most likely answer to this crossword clue is the 14 letter word UTILITARIANISM
I've seen this crossword clue in The Times
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Buddy explains!
The straight part
We know that "Bentham's idea" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case bentham's idea = utilitarianism.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 UTILITARIANISM mean?
UTILITARIANISM noun- A system of ethics based on the premise that something's value may be measured by its usefulness.
- The theory that action should be directed toward achieving the "greatest happiness for the greatest number of people" (hedonistic universalism), or one of various related theories.