The most common and most likely answer to this crossword clue is the 15 letter word FAIR-AND-SQUARE
F
A
I
R
-
A
N
D
-
S
Q
U
A
R
E
Buddy explains!
The straight part
We know that "Honest" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case honest = fair-and-square.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.