Half of a quart (4)
I believe the answer is:
pint
'half of a quart' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for pint that I've seen before include "Imperial measure of capacity" , "Glass of beer (colloq.)" , "Standard measure of Guinness" , "Standard measure of beer or milk" , "Liquid meassure - four gills" .)