The most common and most likely answer to this crossword clue is the 12 letter word CONGREGATION
C
O
N
G
R
E
G
A
T
I
O
N
More clues leading to the result CONGREGATION
- Worshippers go on creating havoc
- Prisoner in rage got out of order in assembly
- Churchgoers go on catering course
- People in the services go on creating mayhem
- Gathering container, go off to collect first of gooseberries
- Flock in ignorance got lost
- Flock mistakenly got grain once
- Those in service go on reacting strangely
- Worshipful company?
- Ringo at once adjusted embracing grand group
Buddy explains!

The straight part
We know that "assembly" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case assembly = congregation.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 CONGREGATION mean?
CONGREGATION noun- The act of congregating or collecting together.
- A gathering of faithful in a temple, church, synagogue, mosque or other place of worship. It can also refer to the people who are present at a devotional service in the building, particularly in contrast to the pastor, minister, imam, rabbi etc. and/or choir, who may be seated apart from the general congregation or lead the service (notably in responsory form).
- A Roman Congregation, a main department of the Vatican administration of the Catholic Church.
- A corporate body whose members gather for worship, or the members of such a body.
- Any large gathering of people.
- A group of eagles.
- (Oxford University) The main body of university staff, comprising academics, administrative staff, heads of colleges, etc.