A cold drink usually hot covered by litres of milk (7)

'of milk' is the definition.
(lacteal means relating to milk)

'a cold drink usually hot covered by litres' is the wordplay.
I cannot really see how this works, but
'a' is present in the answer.
'cold' could be 'c' (eg on taps) and 'c' is found in the answer.
'drink' could be 'tea' (tea is a drink) and 'tea' is found in the answer.
'litres' could be 'll' (the abbreviation for litre written twice) and 'll' is located in the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...

Can you help me to learn more?