The most common and most likely answer to this crossword clue is the 8 letter word SHAKIEST
I've seen this crossword clue in The Times
S
H
A
K
I
E
S
T
Buddy explains!

The straight part
We know that "most unsteady!" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case most unsteady! = shakiest.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 SHAKIEST mean?
SHAKIEST adjective- Shaking or trembling.
- Nervous, anxious.
- (of wood) Full of shakes or cracks; cracked.
- Easily shaken; tottering; unsound.
- Wavering; undecided.