The most common and most likely answer to this crossword clue is the 9 letter word DOGGY-BAG
D
O
G
G
Y
-
B
A
G
Buddy explains!
The straight part
We know that "what diner’s sent home with" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what diner’s sent home with = doggy-bag.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.