The most common and most likely answer to this crossword clue is the 6 letter word BOURNE
I've seen this crossword clue in The Times
B
O
U
R
N
E
More clues leading to the result BOURNE
Buddy explains!

The straight part
We know that "boundary" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case boundary = bourne.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 BOURNE mean?
BOURNE noun- A boundary.
- A goal or destination.
- A stream or brook in which water flows only seasonally.