Spy; functional (9)
I believe the answer is:
operative
I believe this is a double definition.
'spy' is the first definition.
(I've seen this in another clue)
'functional' is the second definition.
(similar in meaning)
(Other definitions for operative that I've seen before include "Functioning; worker" , "secret agent" , "Machinist - working" , "Having force - skilled worker" , "Efficient" .)