Tack on, append (5)
I believe the answer is:
affix
I believe this is a double definition.
'tack on' is the first definition.
(I've seen this in another clue)
'append' is the second definition.
(synonyms)
(Other definitions for affix that I've seen before include "Glue" , "Attach" , "Join" , "Append" , "Fasten (to something)" .)