Open (a fastener) (5)
I believe the answer is:
unzip
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">open a fastener</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 unzip that I've seen before include "Unfasten" , "Loosen fastening" , "Loosen (trousers?)" , "Undo trousers" , "Release covering (or trousers?)" .)