The most common and most likely answer to this crossword clue is the 7 letter word ARCHEST
I've seen this crossword clue in The Guardian
A
R
C
H
E
S
T
More clues leading to the result ARCHEST
Buddy explains!
The straight part
We know that "extremely cunning" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case extremely cunning = archest.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 ARCHEST mean?
ARCHEST verb- To form into an arch shape
- To cover with an arch or arches.
- Knowing, clever, mischievous.
- Principal; primary.