The most common and most likely answer to this crossword clue is the 5 plus 10 letter word GRAND INQUISITOR
I've seen this crossword clue in The Times
G
R
A
N
D
I
N
Q
U
I
S
I
T
O
R
More clues leading to the result GRAND INQUISITOR
Buddy explains!
The straight part
We know that "Question master" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case question master = grand inquisitor.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.