Quantity maybe nearly all gone (6)
I believe the answer is:
gallon
'quantity' is the definition.
(gallon is a kind of quantity)
'maybe nearly all gone' is the wordplay.
'maybe' is an anagram indicator.
'nearly' means to remove the last letter.
'all'+'gone'='allgone'
'allgone' with its final letter taken off is 'allgon'.
'allgon' with letters rearranged gives 'GALLON'.
(Other definitions for gallon that I've seen before include "Texans sometimes wear hats to hold ten of them" , "Eight pints of something" , "Unit of volume for liquid" , "Imperial measure for liquids" , "Quite a few pints" .)