The most common and most likely answer to this crossword clue is the 10 letter word COLLECTION
I've seen this crossword clue in The Daily Telegraph
C
O
L
L
E
C
T
I
O
N
Buddy explains!
The straight part
We know that "offertory" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case offertory = collection.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.
What does COLLECTION mean?
COLLECTION noun- A set of items or amount of material procured or gathered together.
- Multiple related objects associated as a group.
- The activity of collecting.
- A set of sets.
- A gathering of money for charitable or other purposes, as by passing a contribution box for donations.
- Debt collection.
- The act of inferring or concluding from premises or observed facts; also, that which is inferred.
- The jurisdiction of a collector of excise.
- (in the plural, Oxford University) A set of college exams generally taken at the start of the term.
- The quality of being collected; calm composure.