Spy's byword (7)
I believe the answer is:
secrecy
'spy's byword' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for secrecy that I've seen before include "Unwillingness to divulge information" , "Covert activity" , "Concealment" , "Stealth; covertness" , "Suppression of information" .)