Drink on account, opening two litres of milk (7)
I believe the answer is:
lacteal
'of milk' is the definition.
(lacteal means relating to milk)
'drink on account opening two litres' is the wordplay.
'drink' becomes 'tea' (tea is a drink).
'on' says to put letters next to each other.
'account' becomes 'ac' (abbreviation).
'opening' means one lot of letters goes inside another (some letters open a gap in others).
'two litres' becomes 'LL' (the abbreviation for 'litre' repeated).
'tea' after 'ac' is 'actea'.
'actea' placed inside 'll' is 'LACTEAL'.
(Other definitions for lacteal that I've seen before include "Milk-like; pertaining to milk" , "Milky resin" .)