Sodium chloride (4)
I believe the answer is:
salt
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">sodium chloride</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for salt that I've seen before include "Mariner; sodium chloride" , "Crystalline substance used to season food" , "Wit; sodium chloride" , "Common substance" , "Sea dog" .)