Imperial measure (6)
(Other definitions for gallon that I've seen before include "Unit of volume for liquid" , "Several drinks of beer" , "Eight pints of something" , "Imperial measure of capacity" , "8 pints (or ten in a hat!)" .)
'imperial measure' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for ounce that I've seen before include "A light weight - feline" , "Fraction of a pound" , "Lightweight" , "Large cat" , "Atom" .)