Spy’s promise (4)
I believe the answer is:
bond
This clue is a double definition.
'spy's' is the first definition.
(James Bond)
'promise' is the second definition.
(thesaurus)
(Other definitions for bond that I've seen before include "relationship" , "Something that joins together, or a financial document" , "Spy - stick" , "Tie - 007" , "Link, connection" .)