Fort Knox block
I believe the answer is:
ingot
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">fort knox block</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for ingot that I've seen before include "Block of cast metal" , "Precious metal bar" , "Cast metal brick" , "Form of gold or silver" , "Cast metal obtained from a mould" .)