Route is said to be part of an underground network (4)
I believe the answer is:
root
'an underground network' is the definition.
(I've seen this in another clue)
'route is said to be' is the wordplay.
'is said to be' shows a homophone (sound like).
'route' is a homophone of 'ROOT'.
'part of' is the link.
(Other definitions for root that I've seen before include "Origin; eg, carrot" , "Part of plant under the ground" , "radical" , "cheer" , "Rhizome" .)