The most common and most likely answer to this crossword clue is the 6 plus 9 letter word CANATA (CANTATA)
I've seen this crossword clue in The Guardian
C
A
N
A
T
A
(
C
A
N
T
A
T
A
)
Buddy explains!
The straight part
We know that "Religious work" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case religious work = canata (cantata).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.