Spy network given external training? (5)
I believe the answer is:
plant
'spy' is the definition.
(someone planted somewhere to spy)
'network given external training?' is the wordplay.
'network' becomes 'LAN' (Local Area Network).
'given external' indicates putting letters inside.
'training?' becomes 'pt' (Physical Training).
'lan' put into 'pt' is 'PLANT'.
(Other definitions for plant that I've seen before include "Vegetable organism" , "eg Flower" , "workshop" , "Place in the ground to grow" , "Industrial machinery or large vehicles" .)