Underground route (6)
(Other definitions for subway that I've seen before include "New York's "tube"" , "pedestrian passage?" , "crossing point" , "Pedestrian way beneath a road" , "Pedestrian underpass" .)
'underground route' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tunnel that I've seen before include "Passageway under something" , "See 11" , "Excavated passage" , "Underpass" , "See 6" .)