Four quarts (6)
I believe the answer is:
gallon
'four quarts' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for gallon that I've seen before include "Unit equal to 4 quarts" , "Imperial measure of capacity" , "Eight pints of something" , "Eight pints or four quarts" , "Imperial measure for liquids" .)