The most common and most likely answer to this crossword clue is the 6 letter word BUSBOY
I've seen this crossword clue in The Times
B
U
S
B
O
Y
Buddy explains!

The straight part
We know that "Waiter’s assistant" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case waiter’s assistant = busboy.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 BUSBOY mean?
BUSBOY noun- Assistant waiter; one who clears plates from and cleans tables