I'm almost certain that is the coolant return line from a 1.6TD using the water cooled Garrett (T3 maybe). There are 4 tappings into the bearing housing on that turbo. Oil feed / return + Coolant feed / return. A couple of them are banjo type fittings.
The coolant was returned into the frost plug (middle one) at the back of the block, i imagine its a different type of frost plug with provision for the pipe and spring clip.
If its a turbo conversion your doing of some kind, and not using the water cooled Garrett, use a std frost plug and stop the flow of coolant from the block.
Hope this helps. DM