Belt fastener (6)
I believe the answer is:
buckle
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">belt 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 buckle that I've seen before include "Give way to pressure" , "(E.g. belt) fastener; crumple" , "cave in under pressure?" , "Bend - warp" , "Crumple under pressure, or belt fastener" .)