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 before)
(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" .)