The most common and most likely answer to this crossword clue is the 9 letter word SALESGIRL
I've seen this crossword clue in The Guardian
S
A
L
E
S
G
I
R
L
More clues leading to the result SALESGIRL
Buddy explains!

The straight part
We know that "Assistant" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case assistant = salesgirl.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 SALESGIRL mean?
SALESGIRL noun- A young woman employed as a salesclerk.