Diesel return cooler.
In order for the injection system to work properly, there is a hose that returns excess diesel to the tank. That fuel has briefly been heated in the engine compartment, which in theory could be a danger returning to the tank. That's why there is a cooler on the return hose. It got damaged under a van I had, and was leaking. Professional mechanic advised me it can be removed without problem as there is no need to cool the returning diesel, except perhaps if you're driving through the Sahara desert.