The most common and most likely answer to this crossword clue is the 6 plus 4 plus 4 letter word HOLIER THAN THOU
I've seen this crossword clue in The Daily Telegraph
H
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More clues leading to the result HOLIER THAN THOU
Buddy explains!
The straight part
We know that "irritatingly sanctimonious" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case irritatingly sanctimonious = holier than thou.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 HOLIER THAN THOU mean?
HOLIER THAN THOU adjective- Hypocritically or self-righteously pious; sanctimonious.