Eighth of a gallon (4)
I believe the answer is:
pint
'eighth of a gallon' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for pint that I've seen before include "Measure of the plain man's alcoholic drink" , "Liquid measure" , "The working man's beer measure" , "Unit of capacity" , "One eighth of a gallon" .)