The most common and most likely answer to this crossword clue is the 2 plus 4 letter word ON OATH
I've seen this crossword clue in The Daily Telegraph
O
N
O
A
T
H
More clues leading to the result ON OATH
- Old ship builder bites tip of tongue, having sworn )
- Vows having been given to an orderly I didn’t know about
- Bound to speak the truth, old biblical character about Ararat, ultimately
- How a witness may appear to Noah in new guise
- Presumably telling the truth about obscenity
- Oddly deficient, down to last husband? Bound to be honest
- Round closing time, Arkwright’s not — we trust — purveying pork pies!
- Speaking truthfully, we assume, for boat builder, temperature dropping
Buddy explains!
The straight part
We know that "Bound to speak the truth" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case bound to speak the truth = on oath.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.