Union's secret agent (4)
I believe the answer is:
bond
This clue could be a double definition.
'union's' is the first definition.
(thesaurus)
'secret agent' is the second definition.
'bond' can be an answer for 'agent' (James Bond). I'm unsure of the 'secret' bit.
Can you help me to learn more?
(Other definitions for bond that I've seen before include "Fictional agent; join" , "Binding agreement; double zero seven" , "Valuable asset" , "James ...., fiction's 007" , "Fictional agent; link" .)