The most common and most likely answer to this crossword clue is the 8 letter word SURLIEST
S
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R
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T
Buddy explains!

The straight part
We know that "extremely bad-tempered" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case extremely bad-tempered = surliest.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 SURLIEST mean?
SURLIEST adjective- Irritated, bad-tempered, unfriendly.
- Threatening, menacing, gloomy.
- Lordly, arrogant, supercilious.
- In an arrogant or supercilious manner.