The phrase "a unique affair" is correct and usable in written English.
It can be used to describe an event or situation that is distinctive or one-of-a-kind.
Example: "The gala was not just another party; it was truly a unique affair that left a lasting impression on all the guests."
Alternatives: "a one-of-a-kind event" or "a distinctive occasion".
On the one hand, Tempest's literary hip-hop is a unique, genre-defying affair that showcases its author's deft storytelling without ever resorting to showboating.
This wasn't a unique state of affairs — none of the Washington, D.C., publications, including the Post, had levels of diversity commensurate with the city's.
Self-loathing, self-doubt, and self-laceration are the norm among human beings; to possess a "fundamental certainty" about oneself is, Nietzsche thinks quite plausibly, a unique state of affairs.
With a commercial, political or military presence in virtually every corner of the globe, the United States has a unique stake in international affairs.
Non-governmental organizations (NGOs) play a unique role in international affairs, providing access to resources, expertise, and assistance to supplement State resources.
Professor David Kennedy portrays the League as a unique moment when international affairs were "institutionalized", as opposed to the pre First World War methods of law and politics.
Even for those who have never seen it, An Affair holds a unique place in the collective memory of American film-goers, comparable perhaps to the place Brief Encounter has in British hearts and minds.
Kalighat painters often chose to paint mythological themes and Bengali day-to-day life; the paintings on the Tarakeswar affair were a unique exception.
It isn't simply that America, by virtue of symbolizing liberty, has a unique responsibility to shape the affairs of humankind.
On the whole, however, voices from abroad give little comfort to the view that Britain would somehow regain a unique and resonant voice in world affairs once it breaks away from a collective European identity.
