General ambiguity:
* Vague
* Indefinite
* Obscure
* Ambiguous
* Hazy
* Fuzzy
* Uncertain
* Doubtful
* Questionable
Lack of clarity:
* Confusing
* Muddied
* Cloudy
* Blurred
* Ill-defined
* Unintelligible
* Incomprehensible
Lack of information:
* Unknown
* Unspecified
* Undisclosed
* Unrevealed
* Mysterious
* Secret
Lack of understanding:
* Puzzling
* Perplexing
* Confusing
* Baffling
* Mystifying
* Unfathomable
Other:
* Indistinct
* Imprecise
* Unresolved
* Open-ended
* Unsolved
* Unsettled
The best synonym for "unclear" depends on the specific context and what you are trying to convey.