Not fastened (6)
I believe the answer is:
undone
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">not fastened</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 undone that I've seen before include "Ruined - no longer fastened" , "Not finished; untied" , "Ruined or dismantled" , "Annulled; untied" , "Unfastened or brought to ruin" .)