Acetic acid (7)
I believe the answer is:
vinegar
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">acetic acid</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 vinegar that I've seen before include "Liquid used as condiment or food preservative" , "Pickling condiment" , "Ill-temper" , "acidic stuff" , "Peevishness" .)