The most common and most likely answer to this crossword clue is the 10 letter word VEGETARIAN
I've seen this crossword clue in The Daily Telegraph
V
E
G
E
T
A
R
I
A
N
More clues leading to the result VEGETARIAN
Buddy explains!
The straight part
We know that "One strangely negative about a beefburger," is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one strangely negative about a beefburger, = vegetarian.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 VEGETARIAN mean?
VEGETARIAN noun- A person who does not eat animal flesh, or, in some cases, use any animal products.
- An animal that eats only plants; a herbivore.
- Of or relating to the type of diet eaten by vegetarians (in all senses).
- Without meat.
- Of a product normally made with meat, having non-meat substitutes in place of meat.
- (of a person) That does not eat meat.