Quantity some keep in tankard (4)
I believe the answer is:
pint
'quantity' is the definition.
(pint is a kind of quantity)
'some keep in tankard' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'PINT' is hidden in the letters of 'keep in tankard'.
(Other definitions for pint that I've seen before include "Half a quart" , "Unit of capacity" , "The working man's drink measure" , "Imperial measure of capacity" , "Standard measure of Guinness" .)