The most common and most likely answer to this crossword clue is the 9 letter word BUSHWHACK
I've seen this crossword clue in The Guardian
B
U
S
H
W
H
A
C
K
More clues leading to the result BUSHWHACK
Buddy explains!

The straight part
We know that "Turn on" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case turn on = bushwhack.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 BUSHWHACK mean?
BUSHWHACK verb- To travel through thick wooded country, cutting away scrub to make progress
- To fight, as a guerilla, especially in wooded country
- To ambush