The most common and most likely answer to this crossword clue is the 7 letter word ANGRILY
I've seen this crossword clue in Independent
A
N
G
R
I
L
Y
Buddy explains!

The straight part
We know that "In a bitter fashion" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in a bitter fashion = angrily.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 ANGRILY mean?
ANGRILY adverb- In an angry manner; under the influence of anger.