The most common and most likely answer to this crossword clue is the 9 letter word HANDS-OFF
I've seen this crossword clue in The Guardian
H
A
N
D
S
-
O
F
F
Buddy explains!

The straight part
We know that "Non-interventionist" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case non-interventionist = hands-off.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 HANDS-OFF mean?
HANDS-OFF adjective- Tending not to interfere with people's (employees', subjects', etc.) decisions and actions.