The most common and most likely answer to this crossword clue is the 5 letter word ACRID
I've seen this crossword clue in The Daily Telegraph
A
C
R
I
D
More clues leading to the result ACRID
Buddy explains!

The straight part
We know that "bitter" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case bitter = acrid.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 ACRID mean?
ACRID adjective- Sharp and harsh, or bitter and not to the taste.
- Causing heat and irritation.
- Caustic; bitter; bitterly irritating.