The most common and most likely answer to this crossword clue is the 8 letter word FRANKEST
I've seen this crossword clue in The Daily Telegraph
F
R
A
N
K
E
S
T
Buddy explains!
The straight part
We know that "Most open" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case most open = frankest.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 FRANKEST mean?
FRANKEST adjective- Honest, especially in a manner that seems slightly blunt; candid; not reserved or disguised.
- Unmistakable, clinically obvious, self-evident
- Unbounded by restrictions, limitations, etc.; free.
- Liberal; generous; profuse.
- Unrestrained; loose; licentious.