The most common and most likely answer to this crossword clue is the 6 plus 2 plus 3 plus 3 plus 2 plus 4 letter word HONEST AS THE DAY IS LONG
I've seen this crossword clue in Financial Times
H
O
N
E
S
T
A
S
T
H
E
D
A
Y
I
S
L
O
N
G
Buddy explains!
The straight part
We know that "Totally truthful" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case totally truthful = honest as the day is long.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.