After cameoing in Thor, Hawkeye made his full MCU debut in The Avengers, where he transitioned from S.H.I.E.L.D. to mind-controlled Loki goon to one of Earth's Mightiest Heroes. He returned for Avengers: Age of Ultron and decided to hang up his bow at the end to spend more time with his family (including newborn son Nathaniel), but when the kerfuffle surrounding the Sokovia Accords and Bucky Barnes unfolded, Clint was willing to help out once more, which unfortunately resulted in his incarceration. Had Thanos not wiped out half of all life in the universe at the end of Avengers: Infinity War, maybe Clint could have stayed permanently retired from the superhero life, but alas, he's back to shooting arrows at bad guys in Avengers 4. It hasn't been revealed yet specifically why Hawkeye is returning, although one theory posits that his family was among Thanos' victims, which is reason enough for him to suit back up.