The most common and most likely answer to this crossword clue is the 8 letter word LEFTMOST
I've seen this crossword clue in Financial Times
L
E
F
T
M
O
S
T
More clues leading to the result LEFTMOST
Buddy explains!

The straight part
We know that "sinister in the extreme" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sinister in the extreme = leftmost.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 LEFTMOST mean?
LEFTMOST adjective- Furthest to the left.