Fasten; run away (4)
I believe the answer is:
bolt
I believe this clue is a double definition.
'fasten' is the first definition.
(thesaurus)
'run away' is the second definition.
(I've seen this in another clue)
(Other definitions for bolt that I've seen before include "Crossbow missile" , "Short arrow for a crossbow" , "wolf" , "Latch" , "From the blue, it's a great surprise" .)