Good and bad quantity of drink (4)
I believe the answer is:
gill
'quantity of drink' is the definition.
(a gill is a quarter of a pint)
'good and bad' is the wordplay.
'good' becomes 'g' (abbreviation).
'and' means one lot of letters go next to another.
'bad' becomes 'ill' (similar in meaning).
'g'+'ill'='GILL'
(Other definitions for gill that I've seen before include "Respiratory organ of fish" , "A little drink" , "ravine" , "Organ for breathing under water" , "Small liquid measure" .)