Relating to milk (6)
I believe the answer is:
lactic
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">relating to milk</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for lactic that I've seen before include "Obtained from milk" , "Derived from milk" , "The acid in sour milk" , "Kind of acid" , "The acid found in milk" .)