The most common and most likely answer to this crossword clue is the 12 letter word CHEESEBURGER
I've seen this crossword clue in The Guardian
C
H
E
E
S
E
B
U
R
G
E
R
More clues leading to the result CHEESEBURGER
Buddy explains!
The straight part
We know that "fast food" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fast food = cheeseburger.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 CHEESEBURGER mean?
CHEESEBURGER noun- A hamburger containing cheese (usually one or more slices of processed cheese).