The most common and most likely answer to this crossword clue is the 10 plus 3 letter word RIGHT-HAND MAN
I've seen this crossword clue in The Times
R
I
G
H
T
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H
A
N
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M
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Buddy explains!
The straight part
We know that "Assistant who won't be left alongside you?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case assistant who won't be left alongside you? = right-hand man.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.