Unlock (4)
I believe the answer is:
open
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">unlock</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 open that I've seen before include "Allowing access" , "Undo; ready for business" , "Unfasten" , "Undo; ajar" , "Candid or unlocked" .)