The most common and most likely answer to this crossword clue is the 9 plus 3 letter word OFFERTORY BOX
I've seen this crossword clue in The Daily Telegraph
O
F
F
E
R
T
O
R
Y
B
O
X
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.