[4] In 2017, after failing to reach an agreement with the Arizona Coyotes, Clifton signed with the Providence Bruins of the American Hockey League (AHL), where he played 54 games.

[5] He began the 2018–19 season in Providence, and made his NHL debut on November 16, 2018, against the Dallas Stars, after being recalled on an emergency basis.

Clifton was a regular fixture on the third defense pairing for the Bruins until the season was suddenly postponed due to the COVID-19 pandemic, registering two goals in 33 games.

Solidifying himself as an NHL defensemen, Clifton would play in 44 games for the Bruins in the shortened 2020–21 season, scoring a goal and six assists for seven points.

[13] On October 27, Clifton delivered an illegal check to the head on New Jersey Devils captain Nico Hischier, who was injured on the play.