Beer? Mine’s got nitrogen in (4)
I believe the answer is:
pint
'beer?' is the definition.
(beer is typically served by the pint)
'mine's got nitrogen in' is the wordplay.
I cannot quite understand how this works, but
'nitrogen' could be 'n' (N is the chemical symbol for nitrogen) and 'n' is found within the answer.
'mine' could be 'pit' (down the pit) and 'pit' is present in the leftover letters.
No letters remain.
This explanation may well be incorrect...
(Other definitions for pint that I've seen before include "Unit of beer" , "small amount" , "One eighth of a gallon" , "Standard measure of Guinness" , "Measure of the plain man's alcoholic drink" .)