The most common and most likely answer to this crossword clue is the 6 letter word POROUS
P
O
R
O
U
S
Buddy explains!
The straight part
We know that "anything but impregnable" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case anything but impregnable = porous.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 POROUS mean?
POROUS adjective- Full of tiny pores that allow fluids or gasses to pass through.
- With many gaps.
- (by extension) full of loopholes