Bring to ruin: loosen (4)
I believe the answer is:
undo
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">bring to ruin</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >'<span class="explanation-format__original">loosen</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(Other definitions for undo that I've seen before include "Dismantle - destroy" , "Ruin or undermine" , "Unfasten or annul" , "Ruin or loose" , "Overturn" .)