The phrase "a clear notion" is correct and usable in written English.
It can be used when you want to express that someone has a well-defined or easily understood idea about something.
Example: "After discussing the project, I finally have a clear notion of what is expected from me."
Alternatives: "a distinct idea" or "a precise understanding."
But Mr. Dolman has a clear notion of how he sees Christie's brand name versus the competition.
Yet they all speak without a clear notion of people's differential capacities to exercise power in their everyday lives.
Despite his almost childish pettiness, he had a clear notion of what he wanted and how to get it.
Reading Nicholas D. Kristof's column, I wondered whether the American public has a clear notion of the challenge that this condition presents to the clinician.
Mr. Lagerfeld, who has shown some hesitation about fur in recent years, now has a clear notion of his vision, saying, "Fendi is fur — and fur is Fendi".
It was, to all appearances, Cromwell who then came forward with a clear notion of how to achieve Henry's purpose without the Pope.
Law-enforcement officials said they were still seeking a clearer notion of how Mr. Montesinos managed his many enterprises, the exact nature of his relationship with Mr. Fujimori and whether the former spy had more money stashed abroad beyond the $264 million in bank accounts that they have found.
The company speculates that Google Play's requirement of a Google+ user account could have helped diminish the fake reviews problem, because it requires a clearer notion of identity.
That's probably a good thing too, because there have been false starts and detours along the way that might have been more demoralizing if I'd had a clearer notion of how it was supposed to happen.
The Bolsheviks had seized power in 1917 without any clear notion as to how an economy should be run.
He wants to "feature in magazine articles," and grandly imagines a future "retrospective of his work, without having any clear notion of what that work might be".
