Of milk left to turn, I see (6)
I believe the answer is:
lactic
'of milk' is the definition.
(I've seen this in another clue)
'left to turn i see' is the wordplay.
'left' becomes 'L' (common abbreviation).
'to turn' becomes 'act' (theatre).
'see' becomes 'C' (the word for the letter, according to Chambers).
'l'+'act'+'i'+'c'='LACTIC'
(Other definitions for lactic that I've seen before include "Obtained from milk" , "The acid found in milk" , "The acid in sour milk" , "Kind of acid" , "Derived from milk" .)