The most common and most likely answer to this crossword clue is the 12 letter word APPROACHABLE
I've seen this crossword clue in Financial Times
A
P
P
R
O
A
C
H
A
B
L
E
More clues leading to the result APPROACHABLE
Buddy explains!

The straight part
We know that "Open" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case open = approachable.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 APPROACHABLE mean?
APPROACHABLE adjective- Easily approached; easy to talk to.