Ale? Mine’s got nitrogen in (4)
I believe the answer is:
pint
'ale?' is the definition.
(pint of ale)
'mine's got nitrogen in' is the wordplay.
I cannot quite see how this works, but
'nitrogen' could be 'n' (N is the chemical symbol for nitrogen) and 'n' is located in the answer.
'mine' could be 'pit' (down the pit) and 'pit' is found within the remaining letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for pint that I've seen before include "The working man's beer measure" , "The working man's drink measure" , "Unit of beer" , "Standard measure of beer or milk" , "One eighth of a gallon" .)