The most common and most likely answer to this crossword clue is the 12 letter word CHRISTIANITY
I've seen this crossword clue in Financial Times
C
H
R
I
S
T
I
A
N
I
T
Y
Buddy explains!
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.