The most common and most likely answer to this crossword clue is the 6 letter word LABILE
I've seen this crossword clue in The Guardian
L
A
B
I
L
E
More clues leading to the result LABILE
Buddy explains!
The straight part
We know that "Unstable" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unstable = labile.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 LABILE mean?
LABILE adjective- Liable to slip, err, fall, or apostatize.
- Apt or likely to change.
- (of a compound or bond) Kinetically unstable; rapidly cleaved (and possibly reformed).
- (of a verb) Able to change valency without changing its form; especially, able to be used both transitively and intransitively without changing its form.