The most common and most likely answer to this crossword clue is the 7 letter word UNSOUND
I've seen this crossword clue in The Times
U
N
S
O
U
N
D
More clues leading to the result UNSOUND
- A French waterway lacking security?
- A Continental waterway lacking security
- Across the channel, a measure that's flawed
- Duo getting frisky with nuns? That’s doubtful!
- Imperfect result of a French test
- What comes from 12 that we hear is faulty
- A French fathom is not reliable
- Flawed United Nations measure
- A French channel not to be relied on
- Crazy sisters tied up, both topless
Buddy explains!

The straight part
We know that "lacking security" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lacking security = unsound.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 UNSOUND mean?
UNSOUND adjective- Not sound, particularly: