The most common and most likely answer to this crossword clue is the 10 letter word INDEFINITE
I've seen this crossword clue in The Daily Telegraph
I
N
D
E
F
I
N
I
T
E
Buddy explains!

The straight part
We know that "Uncertain" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case uncertain = indefinite.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 INDEFINITE mean?
INDEFINITE noun- (grammar) A word or phrase that designates an unspecified or unidentified person or thing or group of persons or things.
- Without limit; forever, or until further notice; not definite.
- Vague or unclear.
- Undecided or uncertain.
- Being an integral without specified limits.
- Designating an unspecified or unidentified person or thing or group of persons or things