Very secure (4,2,6)
I believe the answer is:
safe as houses
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">very secure</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 safe as houses that I've seen before include "Not in the least at risk" , "Thoroughly secure" , "Not in the least dangerous" .)