A drink for the head (6)
I believe the answer is:
noggin
This clue is a double definition.
'a drink' is the first definition.
(small quantity of alcohol)
'the head' is the second definition.
(noggin is a kind of head)
'for' acts as a link.
(Other definitions for noggin that I've seen before include "Small amount of alcohol" , "Gill usually" , "Drink, small quantity of it" , "Small measure of liquid" , "Measure of alcohol, about a quarter pint" .)