Clasie made his professional debut for Excelsior on 15 August 2010, when he was part of the starting line-up in the Rotterdam derby victory against Feyenoord (3–2). On 31 July 2011 he made his debut for Feyenoord in a friendly game against Málaga. Because of his good play and his tenacity he soon became one of the most popular players among the supporters.
On August 29 2014, a day after Clasie helped Feyenoord qualify for the group stage of the UEFA Europa League, Clasie agreed to extend his contract until 2018. Clasie explained the decision that he was keen on developing himself as a player within the team as well as developing together with the team as a whole.
In November 2010, two months after his debut for Excelsior, he was selected for the Netherlands U21 squad. On February 2012, Bert van Marwijk, the coach of the first team of the Netherlands, pronounced his interest in Clasie. He let him know that he is on the stand-by list for Euro 2012.