Mother's about to nag drunkenly - it's all that champagne! (6)
I believe the answer is:
magnum
'nag drunkenly it's all that champagne' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'mother's about to' is the wordplay.
'mother' becomes 'ma' (Ma is another word for mother or mum).
'about' says the letters should be written in reverse.
'to' becomes 'mung' (I am not sure about this - if you are sure you should believe this answer much more).
'mung' reversed gives 'gnum'.
'ma'+'gnum'='MAGNUM'
Can you help me to learn more?
(Other definitions for magnum that I've seen before include "Double bubbly" , "1 1/2 litre champagne bottle" , "Double-size wine bottle" , "lots of wine" , "Arm" .)