electrolux frig/freezeer?

I have an old Electrolux frig/freezer TR71/55.

It is about 40 years old and has given (and continues to give) excellent service!

However, one fault has developed.

When defrosting (this has to be done manually – it is not an automatic defrost machine) the water does not drain away down what seems to be a small ‘plug’ hole in the middle of the bottom of the freezer. The water is supposed to enter this drain hole , go down a plastic hose and exit at the end of the hose.

Something must be clogging up this plug runaway. It looks as though it can somehow be unscrewed but I don’t want to force anything!

Are you able to give some advice?