Fasten, link (6)
I believe the answer is:
attach
This clue is a double definition.
'fasten' is the first definition.
(I've seen this in another clue)
'link' is the second definition.
(attaching is a kind of linking)
(Other definitions for attach that I've seen before include "Tie, bind to" , "Join; seize (legal)" , "Secure" , "Join to something else" , "Tag on" .)