The most common and most likely answer to this crossword clue is the 5 plus 6 letter word THIRD DEGREE
I've seen this crossword clue in The Times
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More clues leading to the result THIRD DEGREE
Buddy explains!
The straight part
We know that "serious questioning" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case serious questioning = third degree.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 THIRD DEGREE mean?
THIRD DEGREE adjective- Third-degree. Compare first-degree, second-degree and fourth-degree.