The most common and most likely answer to this crossword clue is the 8 letter word PERJURED
I've seen this crossword clue in The Daily Telegraph
P
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Buddy explains!
The straight part
We know that "lied under oath" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lied under oath = perjured.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 PERJURED mean?
PERJURED verb- To knowingly and willfully make a false statement of witness while in court.
- To cause to violate an oath or a vow; to cause to make oath knowingly to what is untrue; to make guilty of perjury; to forswear; to corrupt.
- To make a false oath to; to deceive by oaths and protestations.