Cloak-and-dagger sort (3)
I believe the answer is:
'cloak-and-dagger sort' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for spy that I've seen before include "Enemy agent" , "Glimpse" , "Discover" , "Catch sight of" , "Watch covertly like secret agent" .)