"Indeed it is We who revive the dead and write what they have sent ahead and their effects [which they left behind], and We have figured everything in a manifest Imam."
"Imam mubin" ("manifest imam", "clear imam", or "clear record") is generally understood to refer to a divine record in which all things and divine decrees are inscribed.
(Some exegetes have explained that it is called the "imam" of divine records because it is the highest of the books, hence the leader of books, as opposed to lesser divine records, such as individuals' scrolls of deeds.)
Some hadith also say that the "imam mubin" is Imam Ali. In one hadith, Imam Ali is quoted as saying that, "I am the imam mubin. I distinguish between truth and falsehood, and I have inherited this from the Messenger of Allah."
Perhaps both views are correct, in that it is possible for Allah to provide any of His servants with knowledge of all things.
Some hadith offer explanations for the circumstances of revelation for this verse. However, they do not relate to the phrase "imam mubin" but rather refer to why the previous phrase ("what they have sent ahead and their effects [which they left behind]") might have been revealed. These relate to the community in Medina as well as the general idea that people are rewarded or punished after death for the good or bad practices or legacies they have left behind. However, they do not say anything specific linking the word "imam" directly to the occasion of revelation. I am not aware of any hadith that do this, although there may be some!