Gate fastening (5)
I believe the answer is:
latch
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">gate fastening</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 latch that I've seen before include "Device to hold door shut" , "Light door lock" , "retainer" , "Fastening for door or window" , "Door-catch" .)