Ship’s biscuit (4,4)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">ship's biscuit</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 hardtack that I've seen before include "Former sailor's biscuit" , "Sailors' dry rations" , "Unsalted biscuit, once a ship's staple" .)