My clutch seems like it is lagging when I accelerate?

My clutch seems like it is lagging when I accelerate. When I first drive the car after it's been sitting for a while and it's cooler outside the lagging is less severe. As the vehicle warms up and I drive longer the clutch seems like it slips and the vehicle over revs and doesn't accelerate quickly. I'm sure the clutch is going but do you think it's a belt of some sort that expands when it warms up which makes it slip? I would like to know if I need to replace the whole system or part of it? Thanks for your time.