A cold drink usually hot covered by litres of milk (7)
I believe the answer is:
lacteal
'of milk' is the definition.
(lacteal means relating to milk)
'a cold drink usually hot covered by litres' is the wordplay.
I cannot really understand how this works, but
'a' is within the answer.
'cold' could be 'c' (eg on taps) and 'c' is located 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 remaining letters.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for lacteal that I've seen before include "Milky resin" , "Milk-like; pertaining to milk" .)