Spy in factory (5)
I believe the answer is:
plant
This clue is a double definition.
'spy' is the first definition.
(thesaurus)
'factory' is the second definition.
(I've seen this before)
'in' is the link.
(Other definitions for plant that I've seen before include "Factory; spy" , "Living organism, such as a shrub" , "Machinery; snooker shot" , "Member of vegetable kingdom" , "Place in the ground to grow" .)