The most common and most likely answer to this crossword clue is the 6 letter word BURGER
I've seen this crossword clue in Independent
B
U
R
G
E
R
More clues leading to the result BURGER
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 = burger.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 BURGER mean?
BURGER noun- A hamburger.
- (chiefly as a combining form) A similar sandwich or patty.
- A citizen of a borough or town, especially one belonging to the middle class.
- A prosperous member of the community; a middle class citizen (may connote complacency).