Belt fastener (6)
I believe the answer is:
buckle
'belt fastener' is the definition.
(I've seen this before)
This is the entire clue.
(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" .)