The most common and most likely answer to this crossword clue is the 9 letter word PARALEGAL
I've seen this crossword clue in Independent
P
A
R
A
L
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A
L
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The straight part
We know that "Assistant in brief, say" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case assistant in brief, say = paralegal.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 PARALEGAL mean?
PARALEGAL noun- One who assists a lawyer in routine legal work, but who is without qualified status as a solicitor or barrister (England and Wales), attorney (U.S.), or advocate; known more commonly in the U.S. as a legal assistant.