Strongbox (4)
I believe the answer is:
safe
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">strongbox</span>' is the definition.<br/ ><span class="explanation-format__explanation">(safe is a kind of strongbox)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for safe that I've seen before include "Secure and protected" , "Place for keeping valuables" , "Secure and free from danger" , "Free from harm" , "Secure - place to keep cash" .)