The most common and most likely answer to this crossword clue is the 8 plus 2 plus 4 letter word SERGEANT AT ARMS
I've seen this crossword clue in Financial Times
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More clues leading to the result SERGEANT AT ARMS
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The straight part
We know that "officer trying to keep order" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case officer trying to keep order = sergeant at arms.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.
Crossword Answer definitions
SERGEANT AT ARMS noun- A person charged with keeping order at a meeting.