The most common and most likely answer to this crossword clue is the 8 letter word UNSTABLE
I've seen this crossword clue in The Times
U
N
S
T
A
B
L
E
More clues leading to the result UNSTABLE
- Wobbly: lacking capacity to cross street
- Shetland island qualified as rocky
- A foreign desk admitting head of state’s weak
- Rocky somewhere in Bedfordshire, initially lost
- Regularly furnish item of furniture that’s wobbly
- American acquiring new piece of furniture that’s wobbly
- Dicky heading off from Bedfordshire town
- Dodgy duke removed from Bedfordshire town
- Unhinged duke banished from Beds town
- Rocky islands in Shetlands and Tuscany, where one’s returned
Buddy explains!

The straight part
We know that "weak" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case weak = unstable.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 UNSTABLE mean?
UNSTABLE verb- To release (an animal) from a stable.
- Having a strong tendency to change.
- Fluctuating; not constant.
- Fickle.
- Unpredictable.
- Readily decomposable.
- Radioactive, especially with a short half-life.