Clutch slip on the i-CTDi is usually due to the pressure plate clamping force becoming inadequate, rather than the driven clutch plate becoming worn (which would be the usual way a clutch wears out on other vehicles).
The pressure plate has a self-adjusting mechanism, which means that - to some extent - wear can be compensated for, however this mechanism can be slow to respond... meaning that clutch slip can develop but then recover. Various things can be tried to 'shock' the self-adjusters into action, including using the clutch harshly at take-off a few times, pumping the clutch pedal quickly for a couple of minutes or 'buzzing' the clutch (selecting a inappropriately low gear while travelling at speed and letting the clutch up... MAKE SURE YOU DON'T OVER-REV THE ENGINE WHILE DOING THIS! Or, like me, driving really carefully for a week or two to avoid causing clutch slip while the self-adjusters sort themselves out - my clutch did have a week or so of slipping but has been OK for the last 3 years. Racy Jace is right, sometimes clutch slip can be due to glazing on the drive surfaces, and 'buzzing' or harsh take-offs can help with this as well.
Or your clutch has reached the end of the self-adjustment limit? Clutch replacement on these engines is a common thing, unfortunately.