The most common and most likely answer to this crossword clue is the 9 letter word BITTEREST
I've seen this crossword clue in The Daily Telegraph
B
I
T
T
E
R
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S
T
More clues leading to the result BITTEREST
Buddy explains!
The straight part
We know that "Most hostile" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case most hostile = bitterest.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 BITTEREST mean?
BITTEREST adjective- Having an acrid taste (usually from a basic substance).
- Harsh, piercing or stinging.
- Hateful or hostile.
- Cynical and resentful.