Essentially black cuppa left milky (7)
I believe the answer is:
lacteal
'milky' is the definition.
(I've seen this before)
'essentially black cuppa left' is the wordplay.
'essentially' means to look at the middle letters.
'cuppa' becomes 'tea' (informal term for a cup of tea).
'left' becomes 'l' (common abbreviation).
The centre of 'black' is 'lac'.
'lac'+'tea'+'l'='LACTEAL'
(Other definitions for lacteal that I've seen before include "Milky resin" , "Milk-like; pertaining to milk" .)