Fasten, affix (6)
I believe the answer is:
attach
This clue is a double definition.
'fasten' is the first definition.
(I know that fasten is a more specific form of the action attach)
'affix' is the second definition.
(I know that affix is a more specific form of the action attach)
(Other definitions for attach that I've seen before include "Document carriers" , "Add on to, couple" , "Unite?" , "Join or seize by law" , "'Affix, fasten (6)'" .)