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