Imperial measure of capacity (4)
I believe the answer is:
pint
'imperial measure of capacity' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for pint that I've seen before include "One eighth of a gallon" , "Quantity" , "Measure of the plain man's alcoholic drink" , "Imperial measure of capacity" , "Glass of beer (colloq.)" .)