Works as a spy (5)
I believe the answer is:
plant
This clue is a double definition.
'works' is the first definition.
(synonyms)
'a spy' is the second definition.
(someone planted somewhere to spy)
'as' acts as a link.
(Other definitions for plant that I've seen before include "Living organism without locomotion" , "Factory - bit of vegetation" , "sold by garden centre" , "Factory; establish" , "workshop" .)