The most common and most likely answer to this crossword clue is the 7 letter word VAGUEST
I've seen this crossword clue in The Times
V
A
G
U
E
S
T
More clues leading to the result VAGUEST
Buddy explains!

The straight part
We know that "most uncertain" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case most uncertain = vaguest.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 VAGUEST mean?
VAGUEST adjective- Not clearly expressed; stated in indefinite terms.
- Not having a precise meaning.
- Not clearly defined, grasped, or understood; indistinct; slight.
- Not clearly felt or sensed; somewhat subconscious.
- Not thinking or expressing one’s thoughts clearly or precisely.
- Lacking expression; vacant.
- Not sharply outlined; hazy.
- Wandering; vagrant; vagabond.