The Dutch international has been a regular fixture in Ronald Koeman’s side during their current six-match unbeaten run, which has seen them climb into the top half-dozen of the Barclays Premier League.
Clasie moved to St Mary’s from Feyenoord last summer, but his first months at the club were interrupted by a hamstring problem and then an ankle injury.
The 24-year-old has found his fitness recently, though, and believes his performances are benefiting from that.
“I began with a bad injury and then I came in and it was difficult, because we lost some games and had some draws, but the last few weeks we're doing much better, we have confidence and hopefully I can do it much better in the last 12 games [of the season] than in the last games.
"I play now the last games also 90 minutes, I feel fitter, and the confidence is there in the team. I think that's the most important thing.
“Also for me, as a midfielder, the organisation stays good and I think I can trust everybody in the team and I think that's very important.
"The organisation the last few weeks is very good, so that's nicer for me to play in.”
Clasie admitted the transition to the Premier League has not been a simple one, but the experience is one he has loved.
"It's a hard league, but when I came here I knew that,” he said. “Every game is a hard game and a tough game, so I have to work hard for it, and hopefully I can do it much better than the last weeks.
"Of course, it was for me a dream to play in the Premier League and Southampton gives me the chance, so it's a dream for me.”