Run off to get fastening (4)
I believe the answer is:
bolt
I believe this is a double definition.
'run off' is the first definition.
(I know that run off can be written as bolt)
'fastening' is the second definition.
(bolt is a kind of fastening)
'to get' is the link.
(Other definitions for bolt that I've seen before include "playwright" , "wolf" , "Lightning flash, fasten" , "From the blue, it's a great surprise" , "Type of lightning" .)