Spy link (4)
I believe the answer is:
bond
I believe this clue is a double definition.
'spy' is the first definition.
(James Bond)
'link' is the second definition.
(I know that bond is a more specific form of the action attach)
(Other definitions for bond that I've seen before include "Obligation" , "Luxury shopping area in central London" , "Link based on kinship, marriage or common interest" , "Fictional agent; join" , "Unite" .)