The most common and most likely answer to this crossword clue is the 13 letter word NON-COMMITTAL
I've seen this crossword clue in The Guardian
N
O
N
-
C
O
M
M
I
T
T
A
L
Buddy explains!
The straight part
We know that "On the fence/not a prison sentence" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case on the fence/not a prison sentence = non-committal.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 NON-COMMITTAL mean?
NON-COMMITTAL adjective- Tending to avoid commitment; lacking certainty or decisiveness; reluctant to give out information or show one's feelings or opinion.