The most common and most likely answer to this crossword clue is the 5 plus 6 letter word PLAIN DEALER
I've seen this crossword clue in The Times
P
L
A
I
N
D
E
A
L
E
R
Buddy explains!
The straight part
We know that "One who’s open" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one who’s open = plain dealer.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.