Do you need explicit instructions to enter a runway

Yes, you need an explicit instruction to taxi on any runway.

Note that it doesn’t really matter if the runway is “active” or not. In the eyes of ATC, a runway is always a runway. Don’t expect the controller to say something like “cleared to enter runway xx” – because it is not a clearance, but an instruction. The phraseology used will be along the lines of “back taxi runway xx”, “cross runway xx”, etc. – never “cleared”. The only time you will hear “cleared” and “runway” in the same sentence is in a takeoff clearance or landing clearance.

The only exceptions to this are airports like EGKK that currently have a TFR (Temporary Flight Restriction) stating that runway 08L/26R is to be used a taxiway, for which you would not need a crossing instruction to enter the runway. Stuff like that you need to look out for, but otherwise you need instructions to enter the runway so that you can enter the runway.