The most common and most likely answer to this crossword clue is the 8 letter word TEETERED
I've seen this crossword clue in The Guardian
T
E
E
T
E
R
E
D
More clues leading to the result TEETERED
- Peg, note, taking wine, was unsteady
- Support for driver rising in ranks, not one proved unsteady
- Without hesitation, posted online message, forgetting wife was unstable?
- Support before seen to be held by Irish politician wobbled
- The German retired after support by them oddly wavered
- Deter reshuffles after support for course seesawed
- Duke after having rum unhesitatingly in peg after peg, staggered
- Placed golf ball around tree, swung and moved unsteadily
- Balanced support for ball close to foot before starting to drive
- Youngster almost cleared out the wine and wobbled
Buddy explains!

The straight part
We know that "was unstable" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case was unstable = teetered.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 TEETERED mean?
TEETERED verb- To tilt back and forth on an edge.
- To be indecisive.
- To be close to becoming a typically negative situation.