Drink in a mug (6)
I believe the answer is:
noggin
I believe this is a double definition.
'drink' is the first definition.
(small quantity of alcohol)
'mug' is the second definition.
(noggin is a type of mug or container)
'in a' acts as a link.
(Other definitions for noggin that I've seen before include "Peg" , "Head - tot" , "Measure of alcohol, about a quarter pint" , "Small mug - wooden cup" , "Gill usually" .)