Bent when fastened (7)
I believe the answer is:
buckled
I believe this clue is a double definition.
'bent' is the first definition.
(I've seen this before)
'fastened' is the second definition.
(buckling is a kind of fastening)
'when' is the link.
(Other definitions for buckled that I've seen before include "Fastened up - gave way" , "Crumpled under pressure." , "Bent" , "Securely fastened" , "folded under pressure" .)