In 2017, Clemens made the semi-finals of the World Masters, and missed match darts against the BDO number one Mark McGeeney to reach the final.

He won the fourth day of European Q-School by defeating the Dutchman Vincent Kamphuis 5–3 in the final.

In his first year on the circuit, he reached the final of 2018 PDC Players Championship 11, where he took on two-time world champion Gary Anderson.

In December 2020, he became the first German player to reach the last 16 in the PDC World Darts Championship, after beating reigning champion Peter Wright 4–3.

Firstly, he reached the semi-finals of the 2023 PDC World Darts Championship, becoming the first German player to reach that stage of the competition, before suffering a 6–2 defeat to Michael Smith, having defeated world number one Gerwyn Price in the previous round.