How to do a PARTIAL customs clearance?
If you want to customs clear just one part of a lot, you have to do a split order first for that part of the lot which needs a customs procedure.
A customs procedure can only be done for an entire lot, so if you need just part of it, there is no other way than doing a split order first.
Search + select stock - change dimension - split order

You will get a new lot number for the splitted part which can be now chosen for any customs procedure.