Plenty of possible causes but if you drive conservatively soot tends to accumulate in the exhaust system. Take it out and give it some beans - get the revs over 4k a few times. This is sometimes known as an Italian tune up.
Are there any other symptoms apart from the smoke? My similarly powered Xantia (110) started smoking in the summer - but it was the flat spots and hesitancy I noticed first. Split hose in the turbo/intercooler bit of the induction system.
If the 'Italian' doesn't cure it and you're not finding anything obvious than get an Indy or a diesel specialist to look at it.
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