Fasten again (5)
I believe the answer is:
retie
'fasten again' is the definition.
('tie' is an answer for 'fasten' so perhaps 'retie' is OK for 'fasten again')
This is all the clue.
(Other definitions for retie that I've seen before include "Link up again" , "restore connection" , "Make knot or bow again" , "make another knot" , "Bind again" .)