Fasten or join (6)
I believe the answer is:
attach
I believe this clue is a double definition.
'fasten' is the first definition.
(I know that fasten is a kind of attach)
'join' is the second definition.
(I know that attach is a kind of join)
(Other definitions for attach that I've seen before include "Append, affix" , "Unite?" , "Join or seize by law" , "Fix to something else" , "Join onto" .)