The most common and most likely answer to this crossword clue is the 10 plus 5 letter word ATHANASIAN CREED
I've seen this crossword clue in Independent
A
T
H
A
N
A
S
I
A
N
C
R
E
E
D
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.