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

The straight part
We know that "most sly" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case most sly = leeriest.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 LEERIEST mean?
LEERIEST adjective- Cautious, suspicious, wary, hesitant, or nervous about something; having reservations or concerns.