In Rhineland, it's good red plonk (6)
I believe the answer is:
gutrot
'plonk' is the definition.
(both can mean cheap or poor-quality alcohol)
'in rhineland it's good red' is the wordplay.
I cannot really see how this works, but
'good' could be 'g' (abbreviation) and 'g' is located in the answer.
'red' could be 'trot' (I've seen this before) and 'trot' is located in the answer.
A single letter 'u' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
(Another definition for gutrot that I've seen is " Rough cheap alcohol".)