Spy on the inside (5)
(Other definitions for plant that I've seen before include "Factory equipment" , "workshop" , "Factory; establish" , "eg Flower" , "Factory; organism" .)
'spy on the inside' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for double agent that I've seen before include "Treacherous spy" , "AA man?" .)