The most common and most likely answer to this crossword clue is the 12 letter word PERADVENTURE
I've seen this crossword clue in The Daily Telegraph
P
E
R
A
D
V
E
N
T
U
R
E
More clues leading to the result PERADVENTURE
- Maybe seller rebuffed commercial enterprise
- Prince risks going in both directions, perhaps
- Free trader even up, perhaps
- Queen in flat opening, occasionally hurried possibly
- Excitement must follow for each, maybe
- Maybe for each promotion, enterprise will follow
- Neat catching Queen appearance by chance
- In Peru cut short exciting trip once possibly
- Exciting experience after agent returned by chance
Buddy explains!

The straight part
We know that "by chance" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case by chance = peradventure.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 PERADVENTURE mean?
PERADVENTURE noun- Chance, doubt or uncertainty.
- Perchance or maybe; perhaps; supposing.