The most common and most likely answer to this crossword clue is the 16 letter word HOLIER-THAN-THOU
I've seen this crossword clue in The Guardian
H
O
L
I
E
R
-
T
H
A
N
-
T
H
O
U
More clues leading to the result HOLIER-THAN-THOU
Buddy explains!
The straight part
We know that "Sanctimonious" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case 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.
Crossword Answer definitions
HOLIER-THAN-THOU adjective- Hypocritically or self-righteously pious; sanctimonious.