The most common and most likely answer to this crossword clue is the 13 letter word LAISSEZ-FAIRE
I've seen this crossword clue in The Times
L
A
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Buddy explains!

The straight part
We know that "Not intervening" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not intervening = laissez-faire.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 LAISSEZ-FAIRE mean?
LAISSEZ-FAIRE noun- A policy of governmental non-interference in economic affairs.
- A policy of non-interference by authority in any competitive process.