Clutch slippage can be caused by any number of problems. First would be plate wear, where the friction material wears and the total pack thickness of the combined plates does not produce adequate pressure to fully grab under acceleration. The slippage will often occur in the higher gears and under a load.

The hydraulic clutch system can cause problems with engagement and disengagement.  If the fluid gets old and gritty it can block the return orifice in the master cylinder causing the clutch to not fully engage. Disengagement issues can be caused air in the system, leaks in the system, or clutch hose expansion from a deteriorating hose.

Flushing the system with fresh fluid is a good starting point. Keep in mind that if any of the components are damaged or leaking, the new fluid can make the problem worse.

Use a high quality DOT 4 performance brake fluid. Because the clutch slave cylinder is mounted on the engine, it gets hot. A quality DOT 4 fluid works better in these heated environments.

If you remove the master cylinder cap and find gritty goo, you may find it advantageous to go ahead and rebuild the hydraulic system. This is especially true with the slave cylinder because if it leaks the fluid will attack and ruin the clutch pushrod seal, which is a major repair.