A clutch slips when the friction plate is worn down or contaminated with oil. Symptoms: engine revs rise but the car doesn't accelerate proportionally, especially uphill or in higher gears. Average UK lifespan is 60,000–100,000 miles. Replacement cost: £550–£1,200 at an independent depending on car and whether the dual-mass flywheel needs doing.
Clutch wear is gradual — by the time you notice slip, the friction plate is usually well past its useful life.